Adam Clarke on 2 Corinthians 1:14

Have acknowledged us in part - Απο μερους may signify here not in part, but some of you; and it is evident, from the distracted state of the Corinthians, and the opposition raised there against the apostle, that it was only a part of them that did acknowledge him, and receive and profit by his epistles and advice.

We are your rejoicing, etc. - You boast of us as the ministers of Christ through whom ye have believed; as we boast of you as genuine converts to the Christian faith, and worthy members of the Church of God.