Romans 12:2

And be ye not conformed to this world--Compare Eph 2:2; Gal 1:4, Greek. but be ye transformed--or, "transfigured" (as in Mat 17:2; and Co2 3:18, Greek). by the renewing of your mind--not by a mere outward disconformity to the ungodly world, many of whose actions in themselves may be virtuous and praiseworthy; but by such an inward spiritual transformation as makes the whole life new--new in its motives and ends, even where the actions differ in nothing from those of the world--new, considered as a whole, and in such a sense as to be wholly unattainable save through the constraining power of the love of Christ. that ye may prove--that is, experimentally. (On the word "experience" see on Rom 5:4, and compare Th1 5:10, where the sentiment is the same). what is that--"the" good and acceptable--"well-pleasing" and perfect, will of God--We prefer this rendering (with CALVIN) to that which many able critics [THOLUCK, MEYER, DE WETTE, FRITZSCHE, PHILIPPI, ALFORD, HODGE] adopt--"that ye may prove," or "discern the will of God, [even] what is good, and acceptable, and perfect." God's will is "good," as it demands only what is essentially and unchangeably good (Rom 7:10); it is "well pleasing," in contrast with all that is arbitrary, as demanding only what God has eternal complacency in (compare Mic 6:8, with Jer 9:24); and it is "perfect," as it required nothing else than the perfection of God's reasonable creature, who, in proportion as he attains to it, reflects God's own perfection.

— Jamieson-Fausset-Brown Commentary (public domain)