The most of the brethren (tous pleionas tōn adelphōn). “The more part of the brethren.” The comparative with the article with the sense of the superlative as often in the Koiné.
In the Lord (en Kuriōi). It is not clear whether this phrase is to be connected with “brethren” or with “being confident” (pepoithotas), probably with pepoithotas. If so, then “through my bonds” (tois desmois mou) would be the instrumental case and mean that by means of Paul’s bonds the brethren “are more abundantly bold” (perissoterōs tolmāin).