Romans 12:2 is the central text on transformation in the New Testament, and it opens with a negative: "be not conformed to this world." The Greek syschēmatizesthe — to be squeezed into a mold — is present passive: something is actively trying to shape you into its pattern right now. The world's pressure on your thinking is continuous and structural, not occasional. The antidote Paul offers is equally structural: "be ye transformed by the renewing of your mind." The word for transformed is metamorphoō — the same root as metamorphosis. Not reshaped from outside but restructured from within.
Second Corinthians 3:18 describes the mechanism of transformation: "we all, with open face beholding as in a glass the glory of the Lord, are changed into the same image from glory to glory." The transformation happens through sustained beholding. The Greek katoptrizomenoi means to mirror, to reflect what you are looking at. You become, gradually, what you fix your gaze on. Spiritual formation is largely a question of attention management.
Philippians 1:6 gives transformation a completion date: "He which hath begun a good work in you will perform it until the day of Jesus Christ." The word perform is epitelesei — to bring to full completion, to finish what was started. God does not start renovation projects and abandon them. The transformation you are currently in the middle of has a God-initiated beginning and a God-determined end. You are not a failed project; you are an unfinished one.
Commentary is from a charismatic Protestant perspective, drawing on KJV text and public-domain sources including Spurgeon, Andrew Murray, and Matthew Henry.