Albert Barnes 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. That is, God is true, constant, and will adhere to his promises. He will not deceive. He will not promise and then fail to perform; he will not commence anything which he will not perfect and finish.

Paul's purpose in introducing the idea of God's faithfulness here is to show the reason for believing that the Christians at Corinth would be kept to everlasting life. The evidence that they will persevere depends on the fidelity of God. The apostle's argument is that since they had been called by him into the fellowship of his Son, his faithfulness of character would render it certain that they would be kept to eternal life.

He has presented the same idea in Philippians 1:6: Being confident of this very thing, that he which hath begun a good work in you, will also perform it until the day of Jesus Christ.

You were called.

The word "called" here does not refer merely to an invitation or an offer of life, but to the effectual influence which had been put forth; which had inclined them to embrace the gospel (Romans 9:12).

See Mark 2:17; Luke 5:32; Galatians 1:6–7; Galatians 5:8, 13; Ephesians 1:4; Colossians 3:16. In this sense the word often occurs in the Scriptures, and is designed to denote a power, or influence, that goes out with the external invitation, and that makes it effectual. That power is the agency of the Holy Spirit.

Unto the fellowship of his Son. To participate with his Son Jesus Christ; to be partakers with him (see John 15:1 and following). Christians participate with Christ:

1. In his feelings and views, Romans 8:9.
2. In his trials and sufferings, being subjected to temptations and trials similar to his. 1 Peter 4:13: But rejoice, inasmuch as ye are partakers of Christ's sufferings;Colossians 1:24; Philippians 3:10.
3. In his heirship to the inheritance and glory which awaits him. Romans 8:17: And if children, then heirs; heirs of God, and joint-heirs with Christ; 1 Peter 1:4.
4. In his triumph in the resurrection and future glory. Matthew 19:28: Ye which have followed me, in the regeneration when the Son of man shall sit on the throne of his glory, ye also shall sit upon twelve thrones, judging the twelve tribes of Israel.John 14:19: Because I live, ye shall live also.Revelation 3:21: To him that overcometh will I grant to sit with me in my throne, even as I also overcame, and am set down with my Father in his throne.

From all this, the apostle's argument is that because they partake with Christ in these high privileges, hopes, and promises, they will be kept by a faithful God to eternal life. God is faithful to his Son; and will be faithful to all who are united to him. The argument for the perseverance of the saints is, therefore, sure.