What does this mean? Think of a newborn baby-they can't eat solid food. Switch gears and think of people's spiritual maturity. Paul is comparing the Corinthians spiritual maturity with that of a baby. They were immature. Paul wanted to teach them mature spiritual matter but they wouldn't understand yet, so he withheld. In 3:1, Paul refers to the Corinthian people as "infants in Christ". What made these people immature believers? Paul says in verse 3 that there was "jealousy and strife among them". He also said they were behaving in human ways? The term human here refers to them living out worldly behaviors- ones not of godly action.
6,7 I planted, Apollos watered, but God gave the growth. So neither he who plants nor he who water is anything, but only God who gives the growth.
Paul was addressing the Corinthians here because some of the church was divided in which leader they followed to teach them the gospel. Paul's point here was ultimately God is the One to be glorified, not the man from which the gospel was preached. The men chosen to teach the gospel were merely disciples of the gospel. God is the one creates spiritual growth in people. Therefore, do not take credit for someone deciding to follow Jesus, rather give praise to God alone that you got to be apart of sharing the gospel! God chose you to share Christ, but it is God who internally changes someone's heart. I don't want to discourage you from sharing the gospel- but know that it may take a few planters and waters before growth is seen in someone. In fact you may never see fruit from what you planted. Trust that God chose you to plant a seed, and trust that God, in His divine timing, will produce the growth.
21-23 So let no one boast in men. For all things are yours...whether the world, or life, or death, or the present or the future-all are yours, and you are Christ's, and Christ is God's.
The first sentence plainly summarizes what I was saying about verse 6 and 7. Don't boast in people. To paraphrase this I would say not to boast in anything- in myself, anything I have, anything I might obtain, my future or life itself- for it all belong to God.
For me, this came especially real when my husband and I started giving above our tithe. It was hard at first because we thought we didn't have enough for ourselves. However, when we changed our thinking that everything belonged to God, and what we have has been given to us- it was easier to give.