It is possible to render the latter half of this verse, “that you may test and approve what is the will of God, what is good and acceptable and perfect.” According to the KJV translation, the characteristics of God’s will are described; according to the other rendering, the contents of His will. The essential difference in meaning is slight.

 Gr. dokimazō. This word implies testing and approving. It includes the double process of deciding what the will of God is and then of approving and acting upon it (cf. Rom. 2:18; Eph. 5:10; Phil. 1:10). by the renewing of his mind the believer is enabled to know what God would have him do. He is enlightened to choose between the many perplexing courses of conduct that are offered in this evil age. Since he no longer has a fleshly mind, but the mind of Christ, he is willing to do God’s will, and thus is able to recognize and understand truth (John 7:17). Only the mind that has been renewed by the Holy Spirit can correctly interpret God’s Word. The inspired Scriptures can be understood only by the illumination of the same Spirit by whom they were originally given (see John 16:13, 14; 1 Cor. 2:10, 11; GW 297).

 Before conversion, man’s power of reason, the faculty for discerning between right and wrong, is under the dominion of bodily impulses. The mind is described as a “fleshly mind” (Col. 2:18). But at the time of conversion the mind comes under the influence of the Spirit of God. The result is that “we have the mind of Christ” (1 Cor. 2:13-16). “The words, ‘A new heart also will I give you,’ mean, ‘A new mind will I give you’” (EGW RH Dec. 18, 1913). The death of the old life in the flesh and the beginning of the new life in the Spirit (Rom 6:3-13) is described as “the washing of regeneration, and renewing of the Holy Ghost” (Titus 3:5). This renewing change, which begins when the believer is converted and reborn, is a progressive and continuing transformation, for our “inward man is renewed day by day” (2 Cor. 4:16) “in knowledge” (Col. 3:10). And as the inward man is being transformed by the power of the Holy Spirit, so the outward life is being progressively changed. The sanctification of the mind will reveal itself in a holier way of living, as the character of Christ is more and more perfectly reproduced in the believer (see COL 69).

 Gr. metamorphoō, from which comes our English word “metamorphosis.” In Matt. 17:2; Mark 9:2 it is used to describe the transfiguration of Christ. In 2 Cor. 3:18 it describes the transformation of the believer into the image of Christ. Paul is saying that the Christian should not copy the external and fleeting fashions of this world, but should be thoroughly changed in his inmost nature. Sanctification includes both an outward separation from all the unholy customs of this age and an inward transformation of the believer himself. Elsewhere in the NT this change is described as a new birth (John 3:3), a resurrection (Rom. 6:4, 11, 13), a new creation (2 Cor. 5:17; Gal. 6:15).

 Gr. aiōn, literally, “age” (see on Matt. 13:39; 24:3). Thus “the children of this world” (Luke 16:8; 20:34) may be rendered “the children of this age.” The Christian must not go on following the fashion of this age, as was formerly his habit when he lived according to the flesh (Rom. 8:12). On the contrary he must undergo a complete transformation by the renewing of his mind.

 Gr. suschēmatizō, “to conform oneself to another’s pattern.” The word is translated with the meaning, “to fashion” in 1 Peter 1:14.