Initially, I got this idea for this chapter from Blessed John Henry Cardinal Newman, Lectures on the Doctrine of Justification (1838), chapter 6. Following the lead of his analogy and exegetical argument, I expanded upon it with my usual Scripture study and comparison of words.
The argument, briefly stated, is an analogical one. “Gift” throughout Scripture means something actually imparted internally to us (as in the spiritual gifts). In the overall context of Romans 5, we see a parallel of “made sinners . . . made righteous” (Rom 5:19) — an argument I have used for years — as well as the additional double analogy of “free gift” of both “justification” (5:16) and of “righteousness” (5:17): used synonymously.
The cumulative effect is quite striking, and strongly confirms the Catholic doctrine of actually infused, imparted, intrinsic righteousness / justification and the convergence of justification and sanctification.
The classic Protestant (Calvinist / Lutheran / evangelical) doctrine by contrast, makes justification merely imputed and external and extrinsic, and formally separates sanctification from justification, while stressing that it is still necessary in the Christian life, helpful, and pleasing to God.
Cardinal Newman in the same chapter also makes a brilliant argument, whereby he virtually equates (with much scriptural proof) the Indwelling of the Holy Spirit (and Son and Father) with justification (more on that later). His book is filled with brilliant exegetical insights such as these.
Romans 5:1-21 (entire chapter)
[Bolded] = God’s actions and/or characteristics
[Bracketed] = our actions against God
[Italicized] = our cooperation with God
[Bolded and italicized] = results of God’s actions and our cooperation
Therefore, since we are justified by faith, we have peace with God through our Lord Jesus Christ.  Through him we have obtained access to this grace in which we stand, and we rejoice in our hope of sharing the glory of God.  More than that, we rejoice in our sufferings, knowing that suffering produces endurance,  and endurance produces character, and character produces hope,  and hope does not disappoint us, because God’s love has been poured into our hearts through the Holy Spirit which has been given to us.  While we were still weak, at the right time Christ died for the ungodly.  Why, one will hardly die for a righteous man — though perhaps for a good man one will dare even to die.  But God shows his love for us in that while [we were yet sinners] Christ died for us.  Since, therefore, we are now justified by his blood, much more shall we be saved by him from the wrath of God.  For if while [we were enemies] we were reconciled to God by the death of his Son, much more, now that we are reconciled, shall we be saved by his life.  Not only so, but we also rejoice in God through our Lord Jesus Christ, through whom we have now received our reconciliation.  Therefore as [sin came into the world through one man and death through sin], and so [death spread to all men because all men sinned] –  [sin indeed was in the world before the law was given], but sin is not counted where there is no law.  Yet [death reigned from Adam to Moses] even over those whose sins were not like the transgression of Adam, who was a type of the one who was to come.  But the free gift is not like the [trespass]. For if [many died through one man’s trespass], much more have the grace of God and the free gift in the grace of that one man Jesus Christ abounded for many.  And the free gift is not like the [effect of that one man’s sin]. For the judgment following one trespass brought condemnation, but the free gift following many trespasses brings justification.  If, [because of one man’s trespass, death reigned through that one man], much more will those who receive the abundance of grace and the free gift of righteousness reign in life through the one man Jesus Christ.  Then as [one man’s trespass led to condemnation for all men], so one man’s act of righteousness leads to acquittal and life for all men.  For as [by one man’s disobedience many were made sinners], so by one man’s obedience many will be made righteous.  Law came in, to [increase the trespass; but where sin increased], grace abounded all the more,  so that, [as sin reigned in death], grace also might reign through righteousness to eternal life through Jesus Christ our Lord.
In the same way that we were “made sinners” (the fall and our unregenerate state, which are literal, actual states of being), by Christ’s death we are “made righteous” (a literal, actual state of being rather than a mere abstraction or declaration): Romans 5:19.
“Free gift” is also juxtaposed against the “trespass” of Adam and Eve and ourselves as part of that rebellion (5:15-17). The “free gift” is thus equated in context with justification and being “made righteous.” The “free gift” is, or causes simultaneously all of the following:
1) A gift of grace (5:15, 17).
2) A gift given in grace (5:15).
3) Justification (5:16).
4) Righteousness (5:17).
Righteousness, therefore, cannot be separated from justification; it is intrinsic to it, and we arrive at the Catholic doctrine of infused justification (a thing vehemently opposed by Luther and Calvin and Baptists and other evangelicals today). The absolute proof is in the two related analogies: “made sinners . . . made righteous” (5:19) and “free gift of grace / justification / righteousness.”
All of the following things are asserted in this passage (as a result of Christ’s atoning death and our acceptance of it on our behalf):
1) We’re justified by faith (5:1).
2) We’re justified by His blood (5:9, 16).
3) We’re saved (5:9-10).
4) We’re reconciled to God (5:10-11).
5) We have peace with God (5:1).
6) We stand in an abundance of grace (5:2, 15, 17, 20-21).
7) We have the hope of sharing of the glory of God (5:2).
8) We have hope in general (5:4-5).
9) We have God’s love in our hearts (5:5).
10) We have God in us (the Indwelling) (5:5).
11) We are righteous — equated with justification (5:17, 19, 21).
12) We have acquittal from sin and death (5:18).
13) Eternal life (5:18, 21).
It would seem to follow then, that all of the following are synonymous in essence or at least in large part:
Justification = Salvation by Grace = Reconciliation = Peace With God = God’s Love in Our Hearts = God in Us (Indwelling) = Acquittal = Eternal Life = Righteousness.
Romans 5:21 offers a third related analogy that is akin to the “free grace” analogies of 5:15-17 and “made” analogy of 5:19:
1) Sin used to reign.
2) Now grace reigns through righteousness.
In other words, the same grace that saves and justifies is intrinsically connected with righteousness. It reigns or operates through the righteousness. Therefore, it cannot possibly be formally separated from it. The two things are essentially synonymous.
Grace and righteousness had already been closely connected in 5:17 (“the abundance of grace and the free gift of righteousness”). St. Paul makes another statement of salvation along these lines, connecting Christ’s work, the necessity of our perseverance and chance of falling away, faith, justification, hope, the Holy Spirit, righteousness, and works:
Galatians 5:4-6 You are severed from Christ, you who would be justified by the law; you have fallen away from grace.  For through the Spirit, by faith, we wait for the hope of righteousness.  For in Christ Jesus neither circumcision nor uncircumcision is of any avail, but faith working through love.
Getting back to Cardinal Newman’s argument that inspired this chapter, I shall now examine the use of “gift” itself. Paul elsewhere uses it as a synonym of justification:
Romans 3:24 they are justified by his grace as a gift, through the redemption which is in Christ Jesus,
Romans 6:22-23 But now that you have been set free from sin and have become slaves of God, the return you get is sanctification and its end, eternal life.  For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord.
Protestant theology (for the most part) states that sanctification has nothing to do with salvation. Yet here, eternal life is said to be the “end” of sanctification. That can’t be under Protestant premises. But it makes perfect sense under Catholic premises, whereby sanctification and justification are intertwined or synonymous. Thus, Paul wouldn’t feel any particular need to clarify one against the other. A use of either implies both. That is the case here. Being set free from sin is justification and/or regeneration.
Paul then proceeds to say that we “get” sanctification as a “return.” Therefore, it ineluctably takes place, and in turn leads to eternal life. But in the Protestant view it is optional with regard to salvation. Luther and Calvin urged actual sanctity and adherence to a code of righteousness and morality, but it is formally separated as any means of salvation.
The only problem is that this passage, and many others above and elsewhere do not fit into such a schema. Salvation and justification are free gifts, according to Paul in Romans 6:22-23, and sanctification is in the equation, right along with them. It’s not separated; it is part and parcel.
2 Corinthians 9:14b-15 . . . the surpassing grace of God in you.  Thanks be to God for his inexpressible gift!
Justification is, I submit, implied here.
Ephesians 2:8 For by grace you have been saved through faith; and this is not your own doing, it is the gift of God
Cardinal Newman argued that the word gift in the New Testament (charisma — Strong’s word #5486) always meant a real, internal thing that affected human lives (thus it is also when used for justification and salvation). He wrote in the section mentioned at the top (section 5):
For instance, in Rom. v. 17 we read, “They that receive the abundance of grace, and of the gift of righteousness, shall reign in life by One, Jesus Christ.” The word gift here used certainly must mean a thing given; implying that the righteousness of justification, whatever it turn out to be, is a real and definite something in a person, implanted in him, like a talent or power, and not merely an act of the Divine Mind externally to him, as the forgiveness of sins may be.
But the preceding verses contain a still more convincing statement, on which indeed one might not be unwilling to rest the whole question. St. Paul says, “Not as the offence, so also is the gift … the gift is of many offences unto justification.” Here, observe, he distinctly declares that justification is the result of a gift. Now the word used for “gift” in the original, is the very word used elsewhere for extraordinary gifts, such as of healing, of tongues, and of miracles; that is, a definite power or virtue committed to us. Nowhere else does the word occur in Scripture without this meaning; indeed, it necessarily has it from its grammatical form. For instance, St. Paul says, he “longs to see” the Romans, “that he may impart unto them some spiritual gift;” again, that “the gift of God is eternal life.” He enumerates as gifts, prophecy, ministry, teaching, exhortation, giving, ruling, and showing mercy. Speaking of continence, he says, “Every man has his proper gift from God.” He says, there are “diversities of gifts, but the same Spirit.” He exhorts Timothy “not to neglect the gift that was in him,” but to stir up, to re-kindle, “the gift of God which was in him.” St. Peter too speaks of our “ministering” our “gifts as good stewards.” [Rom. i. 11; vi. 23; xii. 6-18. 1 Cor. vii. 7; xii. 4. 1 Tim. iv. 14. 2 Tim. i. 6. 1 Pet. iv. 10.]
If, then, by a gift is meant a certain faculty or talent, moral, intellectual, or other, justification is some such faculty. It is not a mere change of purpose or disposition in God towards us, or a liberty, privilege, or (as it may be called) citizenship, accorded to us, but a something lodged within us.
This being the case (apart from the analogical exegesis already provided above), if justification is a gift, it is infused, not simply declared in an extrinsic, external, legal fashion.
St. Paul also directly relates the indwelling Holy Spirit to justification and salvation (Newman’s additional argument, alluded to above). Protestants in this instance agree with Catholics that the indwelling is actual and experiential, not merely declared as an abstraction:
1 Corinthians 6:11 And such were some of you. But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and in the Spirit of our God.
Titus 3:5 he saved us, not because of deeds done by us in righteousness, but in virtue of his own mercy, by the washing of regeneration and renewal in the Holy Spirit,
Arguably, he had something similar in mind also, in these additional passages:
Romans 15:13 May the God of hope fill you with all joy and peace in believing, so that by the power of the Holy Spirit you may abound in hope.
1 Corinthians 3:16 Do you not know that you are God’s temple and that God’s Spirit dwells in you?
1 Corinthians 6:19 Do you not know that your body is a temple of the Holy Spirit within you, which you have from God? You are not your own;
1 Corinthians 12:13 For by one Spirit we were all baptized into one body — Jews or Greeks, slaves or free — and all were made to drink of one Spirit.
2 Corinthians 1:22 he has put his seal upon us and given us his Spirit in our hearts as a guarantee.
2 Corinthians 3:6 who has made us competent to be ministers of a new covenant, not in a written code but in the Spirit; for the written code kills, but the Spirit gives life.
2 Corinthians 5:5 He who has prepared us for this very thing is God, who has given us the Spirit as a guarantee.
Galatians 3:14 that in Christ Jesus the blessing of Abraham might come upon the Gentiles, that we might receive the promise of the Spirit through faith.
Galatians 6:8 For he who sows to his own flesh will from the flesh reap corruption; but he who sows to the Spirit will from the Spirit reap eternal life.
Ephesians 1:13 In him you also, who have heard the word of truth, the gospel of your salvation, and have believed in him, were sealed with the promised Holy Spirit,
Ephesians 4:30 And do not grieve the Holy Spirit of God, in whom you were sealed for the day of redemption.
Photo credit: Blessed John Henry Cardinal Newman [public domain]