This explains the faithfulness of Christ toward God. Contrasting Christ with Moses, the writer to the Hebrews urges us to
consider Jesus, the apostle and high priest of our confession, who was faithful to him who appointed him, just as Moses also was faithful in all God’s house. . . . Moses was faithful in all God’s house as a servant, to testify to the things that were to be spoken later, but Christ is faithful over God’s house as a son. (Heb. 3:1–2, 5–6)
The faithfulness of Christ to God is the faithfulness of the Son toward His Father, in pursuit of our salvation according to the terms of the covenant of redemption. This means that whenever we speak of the faithfulness of God in Christ toward us, we are seeing only the tip of the iceberg. The faithfulness of God that we experience in the gospel is the part we can see above the waterline, but underneath this truth, giving it buoyancy, holding it up for us to know and delight in, is the greater part of God’s faithfulness, often unnoticed and overlooked: His faithfulness to Himself and the faithfulness of the Son to the Father in the accomplishment of our redemption for the glory of God’s name.
To change the metaphor, the faithfulness of God to Himself, and of Christ to the Father, provides the deep roots of God’s faithfulness to His covenant people. This glorious, intra-Trinitarian vision of divine fidelity in turn supplies the life of which our faithfulness to God becomes the fruit. We are faithful to God because He is faithful to us. But, as we have seen, He is faithful to us because He is faithful to Himself. He acts for His “name’s sake.”
This is why Christian faithfulness is described as a fruit of the Spirit (Gal. 5:22). Himself the gift of divine faithfulness, the Spirit brings to fruition in us the grace of faithfulness to God. The saints to whom Paul addresses his epistles to the Ephesians and Colossians are called the “faithful in Christ Jesus” (Eph. 1:1; Col. 1:2). Faithfulness marks Christians, not as the ground of their acceptance with God, but as the evidence of it.
Sometimes faithfulness in the New Testament has to do with perseverance. In Acts 11:23, when news of the church begun in Antioch reached Jerusalem, the Jerusalem church sent Barnabas, who “when he came and saw the grace of God, he was glad, and he exhorted them all to remain faithful to the Lord with steadfast purpose.” Similarly, in Revelation 2:10, the church is exhorted to “be faithful unto death, and [Christ] will give you the crown of life.”