Jamieson, Fausset and Brown on Romans 16:19

19. For your obedience--that is, tractableness

is come abroad unto all. I am glad therefore on your behalf--"I rejoice therefore over you," seems the true reading.

but yet I would have you wise unto that which is good, and simple--"harmless," as in Mt 10:16, from which the warning is taken.

concerning--"unto"

evil--"Your reputation among the churches for subjection to the teaching ye have received is to me sufficient ground of confidence in you; but ye need the serpent's wisdom to discriminate between transparent truth and plausible error, with that guileless simplicity which instinctively cleaves to the one and rejects the other."