Jesus encourages his disciples to have more faith.  He criticizes those with little faith (Matt 6:30) and challenges people to believe in a deeper way.  But how do I increase my faith?
 
Growth in faith is a cooperative process.  “Increase our faith”(Luke 17:5) is a prayer, meaning that God holds the key to it.  One man cries to Jesus, “I believe; help my unbelief!”(Mark 9:24).  Yet it is also clear that Jesus wants people to make choices that develop their faith.  “Have faith in God”(Mark 11:22).  “Do not fear, only believe”(Mark 5:36).  God will increase my faith but it is something that I must pursue.
 
I increase my faith by receiving teaching.  I have to be willing to listen.  This is why Jesus often corrects his disciples.  As they seek to be the greatest, Jesus tells them to humble themselves like a child (Matt 18:4).  Jesus corrects them about money, forgiveness, judging, and a host of other topics.  Faith means that I accept correction and trust that Jesus has a better way for me to live.  The more I listen and follow him, the deeper my faith grows.  The challenge is that I do not just learn to know more information about Jesus, but to trust him more.  What am I learning about God right now—and how is he teaching it to me?
 
I increase my faith by developing discernment.  I have to see the spiritual importance of ordinary life.  Jesus is always aware of the spiritual dimensions of the world and tries to open the eyes of his disciples.  “Watch out; beware of the leaven of the Pharisees and the leaven of Herod”(Mark 8:15).  There are moral (not just physical) threats here that must be attended to.  Jesus teaches his followers to make spiritual applications of tragedies and national disasters (Luke 13:1-5) and wants them to interpret “the signs of the times”(Matt 16:3).  Faith means that I see all the events of my life and the wider world through the lens of divine realities.  That includes learning to discern good from evil when they are not always readily apparent (Heb 5:14).  What activity of God and Satan can I discern in my life right now?
 
I increase my faith by consistent, sincere obedience.  Sometimes we obey our way into greater faith.  Peter does this.  He clearly thinks Jesus is foolish to suggest casting the nets again, “but at your word I will let down the nets”(Luke 5:5).  Afterward, his faith is stronger.  When we go ahead and obey despite our misgivings, we often find such efforts rewarded.  I can see no other meaning behind Jesus’ statement, “If you had faith like a grain of mustard seed, you could say to this mulberry tree, ‘Be uprooted and planted in the sea,’ and it would obey you”(Luke 17:6).  While not encouraging us to test God, Jesus wants us to reach out in faith.  The more we obey and expect his blessing, the deeper our faith grows.  We begin to see the strong realities that undergird the promises God has given—and the commands that spring from them.  In what area am I hesitant to obey because I am weak or struggling to understand?  What could I do—even though it is risky or confusing—to rely more strongly on God right now?
 
Then, when we have studied and thought and given our all, we ask for growth from God.  “Increase our faith!”      

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.