‘I adjure you by the Lord that this letter be read to all the brethren.'
The change to the first person probably indicates the point at which Paul took up the pen himself to authenticate the letter. The remainder would have been written by an amanuensis (a kind of secretary). The strength of the request, putting them on oath, suggests that Paul was a little concerned lest otherwise it would not have been read to all. Perhaps he was thinking of some who were out of favour or had separated themselves because of their behaviour. But at this stage it may not have been the custom to read letters in church meetings and Paul may simply have not wanted the letter to be kept to the few.