Paul now writes the Corinthian believers, as usual, before he addresses issues, he reminds them of what Christ has done and admonishes them for the good they are doing. Corinth was a carnal city known for its immorality, people who were known to be immoral were even called "Corinthians." It was also known to be a place of public speaking where scholars and philosophers debated and people followed those they thought were most eloquent with the latest wisdom.
So when Paul wrote that God has called the church to be holy, it was to remind them that they are not supposed to be carnal like the unbelievers around them. When he thanked God for their great ability to teach the Word, he was reminding them that their abilities are not natural but spiritual and God should get all the glory.
Now to the issue at hand, which is division, the Corinthian church was behaving just like the world and comparing Paul, Apollos, Peter, and Christ. Some saying they follow this one while others say they follow that one, arguing about who is more anointed, who can speak the best. Paul rebukes them for such foolishness appealing to them that they live in harmony for Christ is not divided into factions, we are one church, one people, united in Christ. Nobody is anything without Christ, whatever we are is because of the anointed one and His anointing in us. People are not baptized under a particular church or person, they are baptized in the Lord.
We cannot get credit for what comes out of our mouth for we are just vessels of the Holy Spirit. The message of Christ should be simple, that is why those in the world who think they are so wise, reject it. Those who are called by God are often those the world has rejected, the ignorant, the foolish, the unwise, but to them, their hearts are open for it is "the meek that will inherit the earth." "This message offends Jews and the Gentiles think it is nonsense but God uses the so-called foolish of the world to confound the wise, those who are powerless to shame the powerful." "Those who were counted as nothing at all, to use them to bring to nothing that which the world considers important."
So, therefore we must not forget where we came from, and that without God we would be still nothing so our boasting should be in Christ and Christ alone. We need not exalt man like the world does, placing them on a pedestal where they don't belong. We are to honor God by honoring His servants but not in a way that exalts them apart from who they are in God with the understanding that all of us in Christ are part of the same body and therefore should all be honored as His chosen ones.