Three claims surface here from that ubiquitous authority, "They say." First, that the Corinthians belong to Christ in some special way; secondly, that Paul's letters are "frightening" the Corinthians; and finally, that Paul's physical presence is "weak" and his speech "amounts to nothing."
Paul's response is rather firm: "I will not be shamed." (v. 8) Then, in verse 11, "Let such people know that what we say by letter when absent, we will also do when present."
It is unlikely that anything could offend Paul more than the suggestion that someone or some group claims to belong to Christ in a way that other Christians do not. Paul makes the point repeatedly in his letters: There is no ranking of believers when it comes to our relationship with Christ. There is no hierarchy based on spiritual gifts. All baptized believers are equally "in Christ." Any gifts we have are granted by the Spirit as the Spirit has chosen for the best interests of all.
Paul sees his letters as edification, encouraging growth in the churches he founded or visited. His frank speech presupposes a context of friendship and caring. However, not everything that we think is done in love is perceived by the recipients as loving. Since perception is often reality, we must be thoughtful and sensitive at all times, especially when our motives may be obscure or subject to misunderstanding.
When was the last time that you were surprised to discover that your innocent or loving deeds had been perceived as meddlesome, unwanted, and damaging to your relationships?