NIV 1 Corinthians 3:21
So then, no more boasting about human leaders! All things are yours, 22 whether Paul or Apollos or Cephas or the world or life or death or the present or the future—all are yours, 23 and you are of Christ, and Christ is of God.
I believe this is hinting at Christ's coming to do the will of God. He was and is still devoted to the service of his Father. God has a proprietorship in all that he does, since Christ lived, and acted, and reigns to promote the glory of his Father.
John 4:34 Jesus said to them, “My food is to do the will of him who sent me and to accomplish his work."
John 6:38 "For I have come down from heaven, not to do my own will but the will of him who sent me."
The argument here seems to be this, 'You belong to Christ; and he to God. You are bound therefore, not to devote yourselves to a man, whoever he may be, but to Christ, and to the service of that one true God, in whose service even Christ was employed. And as Christ sought to promote the glory of his Father, so should you in all things.'
1 Corinthians 10:31 So, whether you eat or drink, or whatever you do, do all to the glory of God.
Ephesians 4:20-22 20 But that is not the way you learned Christ!— 21 assuming that you have heard about him and were taught in him, as the truth is in Jesus, 22 to put off your old self, which belongs to your former manner of life and is corrupt through deceitful desires, (emphasis added)
Verses are from ESV
The answer here is the contrast Paul makes in the previous discussion about factionalism. Some claimed to be "of Apollos", or, "of Cephas", or, "of Paul" (1 Cor 1:11, 12, 3:4, etc). That is, such people follow the teachings and leadership of their chosen "groupie" (to use a modern term).
Paul draws this section of ant-factionalism to a conclusion by effectively suggesting two things:
- We all should follow (be "of") Christ
- Since Christ of "of God" that this is a much higher (better) group to be part of than mere humans. The NIV & NRSV imply this same idea in their rendering of 1 Cor 3:21-23, "no more boasting about human leaders … you are of Christ and Christ is of God".