John Calvin, Institutes of the Christian Religion (1559 edition, Battles / McNeill), 4.14.18
The term “sacrament,” as we have previously discussed its nature so far, embraces generally all those signs which God has ever enjoined upon men to render them more certain and confident of the truth of his promises. He sometimes willed to present these in natural things, at other times set them forth in miracles.
Here are some examples of the first kind. One is when he gave Adam and Eve the tree of life as a guarantee of immortality, that they might assure themselves of it as long as they should eat of its fruit. Another, we he set the rainbow for Noah and his descendants, as a token that he would not destroy the earth with a flood. These, Adam and Noah regarded as sacraments. Not that the tree provided them with an immortality which it could not give to itself; nor that the rainbow (which is but a reflection of the sun’s rays upon the clouds opposite) could be effective in holding back the waters; but because they had a mark engraved upon them by God’s Word, so that they were proofs and seals of his covenants. And indeed the tree was previously a tree, the rainbow a rainbow. When they were inscribed by God’s Word a new form was put upon them, so that they began to be what previously they were not. That no one may think these things said in vain, the rainbow even today is a witness to us of that covenant which the Lord made with Noah. As often as we look upon it, we read this promise of God in it, that the earth will never be destroyed by a flood. Therefore, if any philosopher, to mock the simplicity of our faith, contends that such a variety of colors naturally arises from rays reflected upon a cloud opposite, let us admit it, and laugh at his stupidity in failing to recognize God as the lord and governor of nature, who according to his will uses all the elements to serve his glory. If he had imprinted such reminders upon the sun, stars, earth, stones they would all be sacraments for us. Why are crude and coined silver not of the same value, though they are absolutely the same metal? The one is merely in the natural state; stamped with an official mark, it becomes a coin and receives a new valuation. And cannot God mark with his Word the things he created, that what were previously bare elements may become sacraments?