John Calvin Commentary Romans 16:19

"For your obedience is come abroad unto all men. I rejoice therefore over you: but I would have you wise unto that which is good, and simple unto that which is evil." — Romans 16:19 (ASV)

Your obedience, etc. This is said to anticipate an objection, for he shows that he did not warn them as if he thought unfavorably of them, but because a fall in their case could easily have happened; as if he had said—"Your obedience is indeed commended everywhere, and for this reason I rejoice on your account: yet, since it often happens that a fall occurs through simplicity, I would have you be harmless and simple concerning doing evil; but in doing good, to be most prudent whenever necessary, so that you may preserve your integrity."

Here we see what that simplicity is which is commended in Christians, so that those who today count their stupid ignorance of the word of God as a high virtue have no reason to claim this distinction. For though he approves that the Romans were obedient and teachable, yet he would have them exercise wisdom and judgment, lest their readiness to believe expose them to impositions. Therefore, he congratulates them because they were free from a wicked disposition; yet he wished them to be wise, to exercise caution.