When St. Paul in his first letter to the Corinthians speaks about marriage, he says “but I tell you it is better”. Why are his personal words in a book that is God’s word?
The questioner refers to verse 25 in the 7th chapter of the 1st letter of St. Paul to the Corinthians (1 Corinthians 7:25; cf. also 1 Corinthians 7:6-35). In this section of his letter St. Paul is not speaking about marriage and virginity in general, but he replies, possibly point by point, to questions that had been put to him. In verses 25 ff Paul answers fiancées who have approached those responsible in the community who in turn have approached him: “I don't know of anything that the Lord said about people who have never been married. But I will tell you what I think. And you can trust me, because the Lord has treated me with kindness.” This answer distinguishes what Paul says from what what the church has received of “what the Lord says”. Such as: The command to remain monogamous, or the command to celebrate the Lord’s Supper in his memory.
The questioner seems to assume that Christians believe St. Paul’s words in his letter to the the word of God in the sense that what Paul wrote was dictated to him by God, independent of the thinking and judgement of the disciple. But this is an understanding of Revelation and of the Word of God that is not known in Christian doctrine.
The questioner is advised to read the relevant sections on this website on the topic: “Holy Scripture and the Word of God”, and in particular III, 1-3. Please also read: Question & Answer No. 236.
The adult catechism Erwachsenen-Katechismus by the German Bishop’s Conference writes:
“The Word of God does not fall from the sky; it reaches us through the human word. That Holy Scripture originates with God means precisely that the individual books within the Bible have human authors. These expressed God’s work in the language of their time, and in accordance with the conditions of their time and their culture, using the literary genres of their time. To interpret Holy Scripture correctly attention has to be paid to what the biblical authors wanted to say, and then to understand what God was trying to tell us through them. It is important to “look carefully at the thought processes, forms of speech and narrative that were rooted in the culture and environment at the time of the authors, and at the forms of human interaction and engagement common in that time” (Die dogmatische Konstitution über die göttliche Offenbarung „Dei Verbum“ [=DV] 12) [The dogmatic constitution on God’s Revelation “Dei Verbum”]. This humanity and historicity of Holy Scripture is a part of what it means for God to come down from heaven, which culminated in the incarnation of Jesus Christ (cf. DV 14).
Part of this humanity and historicity of Holy Scripture is that the individual books of the New Testament were created in the early church for the early church and for the early Christian communities. They are rooted in the life of the early Christian proclamation, liturgy, catechesis, apologetics an in the actual problems associated with organising communities. The early church also collected the various writings of the New Testament and together with the Old Testament declared it to be Canon. Thus the Bible is also a book of the church. It can only be understood correctly if it is interpreted from within the life, the spirit and the faith of the church in which it originated.” (Erwachsenen-Katechismus, Vol 1, p. 48)