When a believer allows it to dominate him. He is literally allowing Satan to rob his entire reason for living.
Immaturity is caused by a lack of grace applied to the soul.
When the grace is not applied to the believer's soul, he lives in his old frame of reference, which could have been formed by rejection, materialism, or covetousness. The believer must remember who he is in Christ to prevent himself from falling into this. Most of all, God has already provided the strength to go on. As you learn to appropriate the grace of God in your life, it will become a shield to you as you grow into maturity. You will not be tempted to draw back because you know that you are being upheld by God's everlasting arms.