John Calvin Commentary Deuteronomy 18:13

"Thou shalt be perfect with Jehovah thy God." — Deuteronomy 18:13 (ASV)

Thou shalt be perfect. He refers to the mutual obligation of that holy covenant by which, as on 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 long for strange religions, by which 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 clarity, namely, 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 remain 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).