Commenting on Ephesians 5:26 Calvin declares: 

When Paul says that we are washed by baptism, his meaning is, that God employs it for declaring to us that we are washed, and at the same time performs what it represents […] Others again suppose that too much importance is given to the sign, by saying that baptism is the washing of the soul […] 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; 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. (Commentary on Ephesians, pp. 319, 320)