John Murray (1898-1975), Scotch-born professor of systematic theology at Westminster Theological Seminary (who surely wasn't as mean as he looks in this picture):
"'A new heart also will I give you, and a new spirit will I put within you' (Ezek. 36:26). God effects a change which is radical and all-pervasive, a change which cannot be explained in terms of any combination, permutation, or accumulation of human resources, a change which is nothing less than a new creation by him who calls the things that be not as though they were, who spake and it was done, who commanded and it stood fast. This, in a word, is regeneration."
"The regenerate person cannot live in sin and be unconverted. And neither can he live any longer in neutral abstraction. He is immediately a member of the kingdom of God, he is spirit, and his action and behaviour must be consonant with that new citizenship. I the language of the apostle Paul, “if any man be in Christ, he is a new creature; the old things have passed away, behold they have become new” (2 Cor. 5:17). There are numerous other considerations derived from the Scripture which confirm this great truth that regeneration is such a radical, pervasive, and efficacious transformation that it immediately registers itself in the conscious activity of the person concerned in the exercises of faith and repentance and new obedience. Far too frequently the conception entertained of conversion is so superficial and beggarly that it completely fails to take account of the momentous change of which conversion is the fruit. . . . Regeneration is at the basis of all change in heart and life. It is a stupendous change because it is God’s recreative act."
"The Holy Spirit is the controlling and directing agent in every regenerate person. Hence the fundamental principle, the governing disposition, the prevailing character of every regenerate person is holiness—he is “Spiritual” and he delights in the law of the Lord after the inward man (I Cor. 2:14, 15; Rom. 7:22). This must be the sense in which John speaks of the regenerate person as not doing sin and as unable to sin (I John 3:9, 5:18)."
--Redemption: Accomplished and Applied (Grand Rapids: Eerdmans, 1955), 96, 104-105, 142.