From Calvin, Institutes, 4.14.3:
"Now, from the definition that I have set forth we understand that a sacrament is never without a preceding promise but is joined to it as a sort of appendix, with the purpose of confirming and sealing the promise itself, and of making it more evident to us and in a sense ratifying it. By this means God provides first for our ignorance and dullness, then for our weakness.
"Yet, properly speaking, it is not so much needed to confirm his Sacred Word as to establish us in faith in it. For God's truth is of itself firm and sure enough, and it cannot receive better confirmation from any other source than from itself. But as our faith is slight and feeble unless it be propped on all sides and sustained by every means, it trembles, wavers, totters, and at last gives way.
"Here our merciful Lord, according to his infinite kindness, so tempers himself to our capacity that, since we are creatures who always creep on the ground, cleave to the flesh, and, do not think about or even conceive of anything spiritual, he condescends to lead us to himself even by these early elements, and to set before us in the flesh a mirror of spiritual blessings."