Romans 2:18

“And knowest his will, etc..] Not the secret will or purpose of God; nor his revealed will in the Gospel, for of this they were ignorant; but his declared will in the law, showing what is to be done, and what is not to be done: to know which in express terms was a privilege, that other people had not; but then the bare knowledge of this will be of no avail: for persons may know their Lord's will, as the Jews did, and not do it, and so be worthy to be beaten with many stripes: and approvest the things that are more excellent: or “triest the things that differ”; from one another, and from the will and law of God; or as the Syriac, “discernest” atylw, “things that are convenient”; agreeable,…”

“approvest the things that are excellent--"triest the things that differ" (Margin). Both senses are good, and indeed the former is but the result of the latter action. (See on Phi 1:10).”

“And knowest his will - The will or commands of God. This knowledge they obtained from the Scriptures; and of course in this they were distinguished from other nations. And approvest - The word used here is capable of two interpretations. It may mean either to distinguish, or to approve. The word is properly and usually applied to the process of testing or trying metals by fire. Hence, it comes to be used in a general sense to try or to distinguish anything; to ascertain its nature, quality, etc.; Luke 12:56. This is probably its meaning here, referring rather to the intellectual process of discriminating, than to the moral process of approving. It could not, perhaps, be said with propriety,…”

“knowest: Deu 4:8; Neh 9:13, Neh 9:14; Psa 147:19, Psa 147:20; Luk 12:47; Joh 13:17; Co1 8:1, Co1 8:2; Jam 4:17 approvest the things that are more excellent: or, triest the things that differ, Phi 1:10 \*marg. Th1 5:21; Heb 5:14 being instructed: Rom 15:4; Psa 19:8, Psa 119:98-100, Psa 119:104, Psa 119:105, Psa 119:130; Pro 6:23; Ti2 3:15-17 Romans 2:19”