When Peter writes "not the putting away of the filth of the flesh" (1 Pet 3:21) in reference to baptism, "he speaks not of the naked sign, but that the effect must also be connected with it... the external symbol is not sufficient except baptism be received really and effectually...
"But the fanatics...absurdly pervert this testimony, while they seek to take away from sacraments all their power and effect. For Peter did not mean here to teach that [baptism] is vain and inefficacious, but only to exclude hypocrites from the hope of salvation, who, as far as they can, deprave and corrupt baptism. Moreover, when we speak of sacraments, two things are to be considered, the sign and the thing itself. In baptism the sign is water, but the thing is the washing of the soul by the blood of Christ and the mortifying of the flesh. The institution of Christ includes these two things. Now that the sign often appears inefficacious and fruitless, this happens through the abuse of men, which does not take away the nature of the sacrament. Let us then learn not to tear away the thing signified from the sign. We must at the same time beware of another evil, such as prevails among the Papists; for as they distinguish not as they ought between the thing and the sign, they stop at the outward element, and on that fix their hope of salvation. Therefore the sight of the water takes away their thoughts from the blood of Christ and the power of the Spirit. They do not regard Christ as the only author of all the blessings therein offered to us; they transfer the glory of his death to the water, they tie the secret power of the Spirit to the visible sign.
"What then ought we to do? Not to separate what has been joined together by the Lord. We ought to acknowledge in baptism a spiritual washing, we ought to embrace therein the testimony of the remission of sin and the pledge of our renovation, and yet so as to leave to Christ his own honour, and also to the Holy Spirit; so that no part of our salvation should be transferred to the sign."
John Calvin, Commentary on the First Epistle of Peter, pp. 118-119.