A.T. Robertson Commentary 1 Corinthians 1:9

"God is faithful, through whom ye were called into the fellowship of his Son Jesus Christ our Lord." — 1 Corinthians 1:9 (ASV)

God is faithful (πιστος ο θεος). This is the ground of Paul's confidence as he loves to say (1 Thessalonians 5:24; 1 Corinthians 10:13; Romans 8:36). God will do what he has promised.

Through whom (δι' ου). God is the agent (δι') of their call as in Ro 11:36 and also the ground or reason for their call (δι' ον) in Heb 2:10.

Into the fellowship (εις κοινωνιαν). Old word from κοινωνος, partner for partnership, participation as here and 2 Corinthians 13:13f.; Php 2:1; 3:10. Then it means fellowship or intimacy as in Ac 2:42; Galatians 2:9; 2 Corinthians 6:14; 1 John 1:3,7. And particularly as shown by contribution as in 2 Corinthians 8:4; 9:13; Php 1:5. It is high fellowship with Christ both here and hereafter.