Now this I affirm and insist on in the Lord: you must no longer live as the Gentiles live, in the futility of their minds. They are darkened in their understanding, alienated from the life of God because of their ignorance and hardness of heart. They have lost all sensitivity and have abandoned themselves to licentiousness, greedy to practice every kind of impurity. That is not the way you learned Christ! For surely you have heard about him and were taught in him, as the truth is in Jesus. You were taught to put away your former way of life, your old self, corrupt and deluded by its lusts, and to be renewed in the spirit of your minds, and to cloth yourselves with the new self, created in the likeness of God in true righteousness and holiness (Eph 4.17-24).
There is very much upon which we could comment here, but I will limit myself to the notion that we are to put off our old selves, our old identities. Becoming a Christian is about becoming a new person. Of course, becoming a new person means that we must lay aside the old person, for which an incredibly important element is forgetting.
I remember youth meetings or services as I was growing up, and one of the more special occasions was when someone who was formerly in the world would give a testimony of how their life changed. They would recall all the things they had done: drugs, alcohol, sex, etc. Then they would publicly announce that all that was behind them, and we would clap and feel good. But I noticed over time that some of those persons who gave public testimonies turned back to their old ways. They were clean for a while but they didn't have the strength to stick with it. Perhaps they didn't cut the ties with their old friends, perhaps they thought of how much enjoyment they had living in the world, or whatever it might have been. In any case, what they didn't do, and what the public testimonies never allowed them to do, is: forget.
Paul puts it like this: Beloved, I do not consider that I have made it [viz., the salvation of Christ and perfection] my own; but this one thing I do: forgetting what lies behind and straining forward to what lies ahead, I press on toward the goal for the prize of the heavenward call of God in Jesus Christ (Phil 3.13-4). We must know what we are aiming for; we must know what this new person we are to become looks like. But at the same time we have to forget what came before, and never think about it. That old person is long dead; a new person is now being born, who knows nothing of the former.
Baptism is a crucial and essential part of this change of identity. Paul says to the Ephesians that we are to be clothed with the new self, made in the image of God. He says to the Galatians that our baptism clothes us with Christ (Gal 3.27), and to the Colossians that Christ is the image of the invisible God (Col 1.15). Our new identity is Christ: we are united to Christ, and we become Christ, in a way.
This means that baptism must be accompanied by forgetting. In a sense, we should understand ourselves to be baptized in the river Lethe, one of the rivers of Hades in Greek mythology which made those who drunk its waters to suffer amnesia. At this juncture I'd like to link you to a song entitled "Lethe" by my favorite band Kayo Dot, the lyrics of which were composed by my friend Tim Byrnes who is a Christian. Notice the theme of forgetting in the lyrics:
All else consumed by a holy cloud of forgetting
Build this man by the way attained
Go forth into the warm waters, brother,
Happy is he, healthy is he among the
pulled out and pulled apart.
He sat with them, but not in vain.
He sat among the ones long-dead,
A feast, a store, a partner, love;
Encourage him in all his ways
To help him to forget the time of disengagement...