In the beginning was the gospel (lower case “gospel”) so that all early Christian teaching was measured by the gospel, but the gospel was known through the apostolic testimony, and the apostolic testimony was known through the writings of the apostles (once they died), leading to this: all Christian theology begins with Scripture. We can finesse “Scripture” in a number of ways — it needs to be interpreted, it derives from the Holy Spirit, it derives from apostolic authority, it is an expression of the regula fidei (rule of faith, earliest confessions) — but when at the bottom all Christian theology begins with Scripture.
What does theology look like if it is Scripture-based? Why is so much “theology” done by discussing what other theologians believe? Have Protestants in effect become Catholic in assuming an authoritative interpretive tradition or at least an authoritative discussion in which theology takes place?
To be sure, the Nicene Creed does not begin with a confession “I believe in the Holy Scriptures, Old and New Testaments.” Yet, without Scripture there is no Nicene Creed since the apostolic testimony, now recorded in Scripture/New Testament, without the Scripture. The Nicene Creed authors were Scripture theologians.
So any worthy theologian needs to articulate a view of Scripture. I confess it as the true Word of God. How about you?
Hence, in Ronald Heine’s Classical Christian Doctrine: Introducting the Essentials of the Ancient Faith, the opening chapter is about Scripture, which gets the subtitle “The Source of Classical Christian Doctrine.” Yet, he knows at Nicea there was no list of which books fit in the NT, there is no official list at this time, there was some discussion… and yet Scripture is at the core of all early Christian theologizing.
What we call Scripture — a book of 27 books in the NT and 39 in the OT — is not what the first Christians would have thought was Scripture. For the first 100 years or so the OT was “Scripture” while the apostolic writings were gaining the same status only because they were already recognized as the Word of God because they witnessed to the gospel. They were the “canon” of the gospel as the gospel was the canon of the 27 books.
The Christian Writings were added to the Scripture (OT) in that the Gospels compelled recognition because they told the Story of Jesus. Paul’s writings reveal three source of authority: the Hebrew Scriptures/OT in Septuagint version, traditions and sayings about Jesus, and apostolic teachings. The Gospels were the first books to be recognized but soon also the apostolic writings. This happened substantively by about 150 to 200.
Two issues provoked serious reflection on which books were authoritative for the Christians: the gnostic claims that books about Jesus they were reading and teaching were true, and the earliest Christian leaders rejected the gnostic claims. Marcion said only Luke and 10 letters of Paul. No Matt-Mark, John, no Peter, no John.
There is a list of NT books called the Muratorian Canon (copy above, text below): Matt-John (Matt and Mark are not listed but it begins with Luke and it says he is the 3d); Acts, Paul’s letters, two rejected forged letters attributed to Paul, Jude, two letters of John, Wisdom of Solomon, and the apocalypses of John and Peter, the author saying he doesn’t like Apoc Peter. The Shepherd is too recent to be accepted. This is probably from the end of the 2d Century.
Scripture, then, shifts from OT only to include the Gospels and then Acts and Paul and the apostles, and all this by the end of the 2d Century, with some nuances and discussions and debates. By Nicene, to be sure, the 27 books are more or less the NT. Regardless, the authority for Christian theology was the Bible — OT and Gospels and the apostles Paul, Peter and John.
. . . at which nevertheless he was present, and so he placed [them in his narrative].  (2) The third book of the Gospel is that according to Luke. (3) Luke, the well-known physician, after the ascension of Christ, (4-5) when Paul had taken with him as one zealous for the law,  (6) composed it in his own name, according to [the general] belief.  Yet he himself had not (7) seen the Lord in the flesh; and therefore, as he was able to ascertain events, (8) so indeed he begins to tell the story from the birth of John. (9) The fourth of the Gospels is that of John, [one] of the disciples. (10) To his fellow disciples and bishops, who had been urging him [to write], (11) he said, ‘Fast with me from today to three days, and what (12) will be revealed to each one (13) let us tell it to one another.’ In the same night it was revealed (14) to Andrew, [one] of the apostles, (15-16) that John should write down all things in his own name while all of them should review it. And so, though various (17) elements [3a] may be taught in the individual books of the Gospels, (18) nevertheless this makes no difference to the faith of believers, since by the one sovereign[3b] Spirit all things (20) have been declared in all [the Gospels]: concerning the (21) nativity, concerning the passion, concerning the resurrection, (22) concerning life with his disciples, (23) and concerning his twofold coming; (24) the first in lowliness when he was despised, which has taken place, (25) the second glorious in royal power, (26) which is still in the future. What (27) marvel is it then, if John so consistently (28) mentions these particular points also in his Epistles, (29) saying about himself, ‘What we have seen with our eyes (30) and heard with our ears and our hands (31) have handled, these things we have written to you?  (32) For in this way he professes [himself] to be not only an eye-witness and hearer, (33) but also a writer of all the marvelous deeds of the Lord, in their order. (34) Moreover, the acts of all the apostles (35) were written in one book. For ‘most excellent Theophilus’  Luke compiled (36) the individual events that took place in his presence — (37) as he plainly shows by omitting the martyrdom of Peter (38) as well as the departure of Paul from the city [of Rome] [5a] (39) when he journeyed to Spain. As for the Epistles of (40-1) Paul, they themselves make clear to those desiring to understand, which ones [they are], from what place, or for what reason they were sent. (42) First of all, to the Corinthians, prohibiting their heretical schisms; (43) next,  to the Galatians, against circumcision; (44-6) then to the Romans he wrote at length, explaining the order (or, plan) of the Scriptures, and also that Christ is their principle (or, main theme). [6a] It is necessary (47) for us to discuss these one by one, since the blessed (48) apostle Paul himself, following the example of his predecessor (49-50) John, writes by name to only seven churches in the following sequence: To the Corinthians (51) first, to the Ephesians second, to the Philippians third, (52) to the Colossians fourth, to the Galatians fifth, (53) to the Thessalonians sixth, to the Romans (54-5) seventh. It is true that he writes once more to the Corinthians and to the Thessalonians for the sake of admonition, (56-7) yet it is clearly recognizable that there is one Church spread throughout the whole extent of the earth. For John also in the (58) Apocalypse, though he writes to seven churches, (59-60) nevertheless speaks to all. [Paul also wrote] out of affection and love one to Philemon, one to Titus, and two to Timothy; and these are held sacred (62-3) in the esteem of the Church catholic for the regulation of ecclesiastical discipline. There is current also [an epistle] to (64) the Laodiceans, [6b] [and] another to the Alexandrians, [6c] [both] forged in Paul’s (65) name to [further] the heresy of Marcion, and several others (66) which cannot be received into the catholic Church (67)— for it is not fitting that gall be mixed with honey. (68) Moreover, the epistle of Jude and two of the above-mentioned (or, bearing the name of) John are counted (or, used) in the catholic [Church];  and [the book of] Wisdom, (70) written by the friends [7a] of Solomon in his honour. (71) We receive only the apocalypses of John and Peter, (72) [7b]though some of us are not willing that the latter be read in church. (73) But Hermas wrote the Shepherd (74) very recently, [7c] in our times, in the city of Rome, (75) while bishop Pius, his brother, was occupying the [episcopal] chair (76) of the church of the city of Rome. [7d] (77) And therefore it ought indeed to be read; but (78) it cannot be read publicly to the people in church either among (79) the Prophets, whose number is complete,  or among (80) the Apostles, for it is after [their] time. (81) But we accept nothing whatever of Arsinous or Valentinus or Miltiades, (82) who also composed (83) a new book of psalms for Marcion, (84-5) together with Basilides, the Asian founder of the Cataphrygians [8a] . . .