Calvin on Psalm 1:
In the second verse, the Psalmist does not simply pronounce those happy who fear God, as in other places, but designates godliness by the study of the law, teaching us that God is only rightly served when his law is obeyed. It is not left to every man to frame a system of religion according to his own judgment, but the standard of godliness is to be taken from the Word of God.
When David here speaks of the law, it ought not to be understood as if the other parts of Scripture should be excluded, but rather, since the whole of Scripture is nothing else than an exposition of the law, under it as the head is comprehended the whole body. The prophet, therefore, in commending the law, includes all the rest of the inspired writings. He must, therefore, be understood as meaning to exhort the faithful to the reading of the Psalms also.
From his characterizing the godly as delighting in the law of the Lord, we may learn that forced or servile obedience is not at all acceptable to God, and that those only are worthy students of the law who come to it with a cheerful mind, and are so delighted with its instructions, as to account nothing more desirable or delicious than to make progress therein. From this love of the law proceeds constant meditation upon it, which the prophet mentions on the last clause of the verse; for all who are truly actuated by love to the law must feel pleasure in the diligent study of it.