From John Calvin's commentary on Galatians 3:27.
"The greater and loftier the privilege is of being the children of God, the farther is it removed from our senses, and the more difficult to obtain belief. He therefore explains, in a few words, what is implied in our being united, or rather, made one with the Son of God; so as to remove all doubt, that what belongs to him is communicated to us. He employs the metaphor of a garment, when he says that the Galatians have put on Christ; but he means that they are so closely united to him, that, in the presence of God, they bear the name and character of Christ, and are viewed in him rather than in themselves. This metaphor or similitude, taken from garments, occurs frequently, and has been treated by us in other places.
"But the argument, that, because they have been baptized, they have put on Christ, appears weak; for how far is baptism from being efficacious in all? Is it reasonable that the grace of the Holy Spirit should be so closely linked to an external symbol? Does not the uniform doctrine of Scripture, as well as experience, appear to confute this statement? I answer, it is customary with Paul to treat of the sacraments in two points of view. When he is dealing with hypocrites, in whom the mere symbol awakens pride, he then proclaims loudly the emptiness and worthlessness of the outward symbol, and denounces, in strong terms, their foolish confidence. In such cases he contemplates not the ordinance of God, but the corruption of wicked men. When, on the other hand, he addresses believers, who make a proper use of the symbols, he then views them in connection with the truth -- which they represent. In this case, he makes no boast of any false splendor as belonging to the sacraments, but calls our attention to the actual fact represented by the outward ceremony. Thus, agreeably to the Divine appointment, the truth comes to be associated with the symbols.
"But perhaps some person will ask, Is it then possible that, through the fault of men, a sacrament shall cease to bear a figurative meaning? The reply is easy. Though wicked men may derive no advantage from the sacraments, they still retain undiminished their nature and force. The sacraments present, both to good and to bad men, the grace of God. No falsehood attaches to the promises which they exhibit of the grace of the Holy Spirit. Believers receive what is offered; and if wicked men, by rejecting it, render the offer unprofitable to themselves, their conduct cannot destroy the faithfulness of God, or the true meaning of the sacrament. With strict propriety, then, does Paul, in addressing believers, say, that when they were baptized, they "put on Christ;" just as, in the Epistle to the Romans, he says,
"that we have been planted together into his death, so as to be also partakers of his resurrection."(Romans 6:5.)"In this way, the symbol and the Divine operation are kept distinct, and yet the meaning of the sacraments is manifest; so that they cannot be regarded as empty and trivial exhibitions; and we are reminded with what base ingratitude they are chargeable, who, by abusing the precious ordinances of God, not only render them unprofitable to themselves, but turn them to their own destruction!"