Calvin interprets the “washing of water” in Ephesians 5:26 as a reference to baptism, and goes into a little digression on baptism.
Paul is telling us “that we are washed by baptism,” and by this he means “that God employs it for declaring to us that we are washed, and at the same time performs what it represents.” The water and the rite are in themselves nothing, and we should put no confidence in the work of a minister or in the element of water: “If the truth – or, which is the same thing – the exhibition of the truth – were not connected with baptism, it would be improper to say that baptism is the washing of the soul.” But, he implies, when the exhibition of the truth is connected with the rite, it is entirely proper to call it the “washing of the soul.”
As he goes on to explain,
“Others again suppose that too much importance is given to the sign, by saying that baptism is the washing of the soul. Under the influence of this fear, they labour exceedingly to lessen the force of the eulogium which is here pronounced on baptism. But they are manifestly wrong [note – not just wrong, but manifestly so!]; for, in the first place, the apostle does not say that it is the sign which washed, but declares it to be the work of God. It is God who washes, and the honour of performing it cannot lawfully be taken from its Author and given to the sign. But there is no absurdity in saying that God employs a sign as the outward means. Not that the power of God is limited by the sign, but this assistance is accommodated to the weakness of our capacity. Some are offended at this view, imagining that it takes from the Holy Spirit a work which is peculiarly his own, and which is everywhere ascribed to him in Scripture. But they are mistaken [though, apparently, not manifestly mistaken]; for God acts by the sign in such a manner, that its whole efficacy depends upon his Spirit. Nothing more is attributed to the sign than to be an inferior organ, utterly useless in itself, except so far as it derives its power from another source.”
This is no inhibition of the freedom of God either: “The grace of God is not confined to the sign; so that God may not, if he pleases, bestow it without the aid of the sign. Besides, many receive the sign who are not made partakers of grace; for the sign is common to all, to the good and to the bad alike; but the Spirit is bestowed on none but the elect, and the sign, as we have said, has no efficacy without the Spirit.” So, God is free to use the sign to bestow the reality, but He is also free to withhold the reality even when the sign is given.