D-R Bible, Haydock Commentary:
Ver. 14. Defence. Jonathas and Chusai saved David.
Ver. 16. And immortality, is not in Greek. (Calmet) — But shews the meaning of life is this place; as a true friend will not cease to give good advice for eternity. (Haydock) — But even in this world, nothing can be more advantageous. (Calmet) — Amicus magis necessarius est quam ignis et aqua. (Cicero) (St. Ambrose, off iii.) — Him. Cicero himself says, “friendship can subsist only among the virtuous.” Yet these, judging others by themselves, are more easily imposed upon, and ought, therefore, to address themselves to God. (Calmet)
Ver. 17. Be. He will instill into his friend sentiments of piety, if he have them not before. Amicitia similes invenit aut facit. “Pythagoras desires that in friendship one should be formed of many.” (Cicero v. 11.) (Haydock)