The prayer you never outgrow is found in Luke 18:13. Jesus is telling a story about a Pharisee and his prayer, where he celebrates how great he is and how much better he is than other people.
Then the tax collector prays. “But the tax collector, standing far off, would not even lift up his eyes to heaven, but beat his breast, saying, ‘God, be merciful to me, a sinner’”(Luke 18:13). Jesus goes on to say that this man went down to his house justified rather than the other. He prays, "God, be merciful to me, a sinner"--and he means it.
I believe that this spirit of humility—what Jesus calls being poor in spirit—is the most important beginning place for discipleship. We have to understand that we are in need of God’s mercy and we come to God not as free agents, wondering what the benefits are if we join his team, but as beggars and sinners.
But I stress that this is a prayer you never outgrow because there is a danger that what begins in humility sometimes hardens into pride, like that Pharisee.
Paul says that “Christ Jesus came into the world to save sinners, of whom I am the foremost”(1 Tim 1:15).
Paul is thankful that God gave him a ministry “though I am the very least of all the saints”(Eph 3:8).
I understand that Christians have become different people. We are born again. We have a past, but that past no longer defines us or describes us. Such were some of you (1 Cor 6:11). And I understand that Paul talks about forgetting the past and reaching forward to the future (although he’s really talking about the self-righteousness he has left behind, Phil 3:13). Yet I still believe that there is a place for us to pray “God be merciful to me a sinner.” It’s not a matter of describing our current pattern of life, but remembering where we have come from and how unworthy we are. No matter how much we grow in Christ and accomplish through him, we are still unworthy of God’s approval.
There is such a thing as a healthy view of our past—not wallowing in sin, but not forgetting either. That’s what this prayer helps us to have. So this prayer is one that typifies Christians. If I find I cannot say this—or that it lacks the intensity it deserves—or that I would rather recount all my good things—then I have become spiritually sick.
In fact, it might be good to tie it together with a kind of prayer from Luke 17: “So you also, when you have done all that you were commanded, say, ‘We are unworthy servants; we have only done what was our duty’”(Luke 17:10). Discipleship begins and ends in humility--and our prayers should reflect it.