Calvin has a fascinating explanation of the sacraments in his Institutes of the Christian Religion. He introduces the sacraments as signs and seals, as the Reformers usually do. But, he goes on to explain the sacraments as external words which affix our faith and deepen our union with Christ by the Spirit.
Calvin explains it this way:
First, we must attend to what a sacrament is. It seems to me, then, a simple and appropriate definition to say, that it is an external sign, by which the Lord seals on our consciences his promises of good-will toward us…We may also define more briefly by calling it a testimony of the divine favour toward us, confirmed by an external sign, with a corresponding attestation of our faith towards Him. (The Institutes of the Christian Religion, 14.1)
Calvin speaks of a sacrament of an “external sign” of God’s good will toward us in Christ which he inwardly “seals” on our consciences to sustain us the “weakness of our faith”. It is God showing us visibly the promise of good will to us in the gospel. Baptism, properly speaking then, is God sealing upon our consciences his promise of renewal by the Spirit; the Lord’s Supper, that are welcome to his table.
Calvin goes on to say this:
In this way God provides first for our ignorance and sluggishness and, secondly, for our infirmity; and yet, properly speaking, it does not so much confirm his word as establish us in the faith of it. For the truth of God is in itself sufficiently stable and certain, and cannot receive a better confirmation from any other quarter than from itself. But as our faith is slender and weak, so if it be not propped up on every side, and supported by all kinds of means, it is forthwith shaken and tossed to and fro, wavers, and even falls. And here, indeed, our merciful Lord, with boundless condescension, so accommodates himself to our capacity, that seeing how from our animal nature we are always creeping on the ground, and cleaving to the flesh, having no thought of what is spiritual, and not even forming an idea of it, he declines not by means of these earthly elements to lead us to himself, and even in the flesh to exhibit a mirror of spiritual blessings. (14.3).
Calvin says rightly that the “truth of God is in itself sufficient”, but because we are physical beings, God condescends to us by physical means to “prop our faith up on every side”. The sacraments are physical words to us, to accommodate our bodily situation. Sacraments are not properly sacraments without the words or promises contained under the external sign.
In fact, for Calvin, the word and sacrament accomplish the same thing:
Both word and sacraments…confirm our faith, bringing under view the kind intentions of our heavenly Father, in the knowledge of which the whole assurance of our faith depends, and by which its strength is increased; and the Spirit also confirms our faith when by engraving that assurance on our minds, he renders it effectual. (14.11)
So then, sacraments are physical words to us, signs and seals, to bolster our faith, to deepen our union with Christ by the action of the Spirit. They are Spirit wrought instruments to seal to our consciences God’s good promises to us by the gospel.