It is through Baptism that the Father claims us as His children and by Grace plants in us the seeds of Faith.
Such seeds need nourishment throughout our lives for we live in the world, (subject to sin) but are not of the world (for God has claimed us). Martin Luther aptly describes this condition of the Christian life as living as "simultaneously saint and sinner." God continually calls us to turn from sin as He feeds our Faith trough the confession and forgiveness of sins, preaching of the Law and Gospel and partaking of Christ's Body and Blood in the Holy Supper.
A full description of the Christian faith in the Lutheran tradition can be found in the writings of the Book of Concord and Augsburg Confessions.