1 Corinthians 1:9

God is faithful, by whom ye were called
These words contain arguments, assuring the saints of their confirmation in grace, and of their being preserved blameless to the day of Christ, taken from the faithfulness of God, who is always true to his promises: whatever he has said, he will do it; he will never suffer his faithfulness to fail; and since he has made so many promises concerning the establishment of his people, and their perseverance to grace, they may assure themselves of them; and also from his having called them by his grace, for whom he effectually calls by his grace, he glorifies; and particularly from his having called them

unto the fellowship of his Son Jesus Christ our Lord;
to partake of his grace, and to be heirs of glory with him; to enjoy communion with him in private and public exercises of religion, which is an evidence of being in him, and of union to him; for it is not merely into the fellowship of his saints or churches, but into the fellowship of his Son they are said to be called; and such are members of Christ, of his body, of his flesh, and of his bone; and shall never be lost and perish, but shall be confirmed to the end; be preserved in him blameless, and presented to him faultless, and have everlasting life.