The term comes from the reference in Romans 8.
We know that God causes all things to work together for good to those who love God, to those who are called according to His purpose. For those whom He foreknew, He also predestined to become conformed to the image of His Son, so that He would be the firstborn among many brethren; and these whom He predestined, He also called; and these whom He called, He also justified; and these whom He justified, He also glorified. (Rom 8:28-30).That last term is in view here. It is a certainty. Those whom He called ... He also glorified. No doubt. No contingencies. No question. But ... what is it?
Glorification refers to the end of the story. What happens to the believer in the final outcome? Part of that story is "the hope of eternal life" (Titus 3:7). Another is what theologians refer to as "the Beatific Vision", of which John wrote when he said, "Beloved, we are God's children now, and what we will be has not yet appeared; but we know that when He appears we shall be like Him, because we shall see Him as He is" (1 John 3:2). It is the final step, the last transformation, that ultimate union between believers and Christ. We will be changed. New bodies. New beings. No longer capable of sin.
This glorification is based on Christ's return. This is what is referred to as eschatology, the study of last things. There is a lot of disagreement within Christianity about the details of eschatology. Will there be a "Rapture" or won't there? When will that Rapture occur? Will there be a literal 1,000 year millenial reign or won't there? How literal is Revelation? Oh, lots and lots of questions. But there are a few certainties about which there is no disagreement. One fundamental fact is that Christ will return. He promised it. He will do it. End of question.
In Christ's return, Christians will have their final change (1 Thess 4:16-17). At that point, we are told, our bodies will be raised. Christianity does not teach that our spirits are imprisoned in some sense in our bodies and, in the end, we'll be free from them (1 Cor 15:22-23). No, our bodies will be raised.
Behold! I tell you a mystery. We shall not all sleep, but we shall all be changed, in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised imperishable, and we shall be changed. For this perishable body must put on the imperishable, and this mortal body must put on immortality (1 Cor 15:51-53).Glorification refers to that moment at Christ's return when we will be changed, when we will see Him as He is and be like Him. We will no longer be capable of sin, no longer be capable of sadness. We will be in perfect union with Him and not merely as disembodied spirits, but as physical beings in glorified bodies (Phil 3:20-21).
Saved from wrath is a good thing. Saved by grace through faith is wonderful. Imputed righteousness is great. Being transformed into the image of Christ is marvelous. But it is this final step at the return of our Savior that the entire story is culminated. It is the final outome of God's perfect plan. It is part of the basic essence of Christianity.