Charles Spurgeon 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, by whom you were called to the fellowship of his Son Jesus Christ our Lord.

As Paul wrote to the Thessalonians, Faithful is he that calleth you, who also will do it. To be called by the faithful God is the guarantee of everlasting salvation.