Paul had known the Corinthian church for quite some time when he had com to write the letter we refer to as 1 Corinthians. In actuality this is likely the second letter Paul wrote to this church as Paul makes reference to some other letter he had previously sent in 1 Cor. 5:9. The issue of the missing letter ought not erode the understanding of the authority of scripture. For a variety of reasons the early church viewed this letter as an authoritative document which should shape the wider church and so preserved and made copies of it. The fact that they did not so view Paul's earlier letter need not discredit Paul for the contemporary reader.
Paul commends the church in his prayer, thanking God that they have been transformed and "enriched in him in all speech and all knowledge" (1 Cor. 1:5). The content of this prayer is one of the keys to the interpretation of the letter as Paul discusses division, lack of knowledge, lack of maturity and moral failure. Paul weaves between referring to the issues in the Corinthian church and the things that have been taught to them. Paul is holding the people up to the things they claimed to believe, holding them to account.
The principle issue of belief follows the opening issue of divisions. Paul responds to church division by reminding the Corinthians that they believed a 'foolish' message of a crucified Messiah. Therefore they should not be divided between themselves as though the Apostles are competing teachers of philosophy. Through faith the Corinthians are already separated from the world. There ought not be any further division.
This idea of the separateness of Christian thought and life dominates this letter. The church is called to be a colony of people who are different from the world and need nothing from the world but rather take care of their own affairs that they might serve the purposes of God. Since the Corinthians are unable to do this Paul views them as immature.
The separateness Paul implies is not supposed to create a hostility with the world, rather it is a logical outworking of belief in Jesus, in his death and resurrection. Since Jesus challenged the wisdom and the ways of the world by rising from the grave, Paul yearns to see the new people of God living in light of this most foolish and unbelievable thing.
Responding to issues of unity, immorality and relations with the pagan world Paul keeps reminding the Corinthians of the uniqueness of the message they have heard and the uniqueness of the Apostles who brought it to them, how this message they had heard is unlike any human wisdom and how the Apostles have been unlike any human teachers, laboring together to build up the people of God.