Why can't we expect new believers to be mature right away?

Les explains that the spiritual life is a parallel with the physical. Just as no infant is born mature, no one is born into God's family as a full, mature spiritual person; all enter salvation as babes in Christ. He illustrates this with an old evangelist who, traveling by train, could tell which homes had babies by diapers hanging on clotheslines. In the same way, infant believers will have failures. No infant is trained immediately, so we cannot expect new believers to suddenly be mature or to have the same spiritual discoveries as longer, older Christians. The Corinthians were saved out of abject paganism, idolatry, and immorality, so they needed a time of nurturing and growth, but only by God's grace. Every saved person experiences all of God's grace at salvation, yet still has a long way to go in growth. As Peter says, believers grow in grace; they do not grow into grace. Grace is not law, so nothing is forced upon them.