Matthew Poole and continuators on 1 Corinthians 1:9

9 God is faithful , by whom ye were called unto the fellowship of his Son Jesus Christ our Lord .

Faithfulness in the same with Veracity or Truth to a mans word, which renders a person fit to be credited. It is a great Attribute of God, Chap. 10▪ 13. 1 Thess. 5.24. This implieth promises of God for the perseverance, of Believers (of which there are many to be found in holy Writ.

But these promises concern not all, but such only whom God hath chosen out of the world, calling them to a communion with Christ, which necessarily supposeth union with him. So as here is another argument to confirm them, that God would keep them to the end, so as they should be blameless in the day of Christ. Because God had called them into that state of Grace, wherein they were, and would not leave his work in them imperfect, he had called them into the fellowship of Iesus Christ. See 1 Iob. 1.3. into a state of friendship with Christ; and into a state of union with him, into such a state, as he would daily by his Spirit be communicating the blessed influences of his Grace unto them.

Marginal references: 2 Thess. 3