John Calvin on Deuteronomy 18:13

13. Thou shalt be perfect. He refers to the mutual obligation of that holy covenant whereby as on the one side He had pledged Himself to the Jews, so on the other He had made them His debtors, not to prostitute themselves to idols, or to hanker after strange religions, whereby men’s minds are led astray. This perfectness, then, is opposed to all those mixtures or corruptions which withdraw us from the sincere worship of the one true God; because the simplicity which retains us in obedience to heavenly teaching, is that spiritual chastity which God requires in His Church. The context of the passage proves this with sufficient clearness, viz., that God would restrain the Jews from all licentiousness, so that being devoted to His service, they should not look this way or that way, nor be carried away by vanity and instability, but constantly abide in the pure worship which He had prescribed to them. For this reason Paul declares that he is jealous for Christ; and because he had “espoused” the Corinthians to Christ, he feared “lest by any means, as the serpent beguiled Eve, through his subtlety,” so being ensnared by the wiles of impostors, they should fall “from the simplicity that is in Christ.” (2 Corinthians 11:2.)