Understanding that sin makes us thick-headed, unwilling to accept certain truths when they receive minimal emphasis, the biblical authors repeat foundational doctrines so that they might penetrate our hearts and minds. The man-made religions of this world prove that without the work of the Holy Spirit, people think that they are basically good and can contribute something to their salvation. This strips glory from God and gives it to us, for if we can do even one thing to merit salvation, then we deserve some credit. All belief systems except biblical Christianity encourage us to believe that we contribute to our salvation, even if they deceitfully assert otherwise.
It should have been clear from reading Ephesians 2:1–7 that the Lord owes us nothing but justice and wrath. (Moreover, this is taught throughout the Bible: Gen. 6:5–7; Deut. 7:6–11; Ps. 51:4; Rom. 1:18–3:20; James 2:10–11). But in case we missed it, Paul in Ephesians 2:8–10 tells us one more time that we bring nothing with us to our redemption, and that any good works we perform are not the ground of our status before God but result from us having been chosen and gifted with salvation: “By grace you have been saved through faith. And this is not your own doing; it is the gift of God” (Eph. 2:8). Grace, by definition, excludes the slightest hint that human merit contributes to our righteous standing before the most holy and perfect Creator, and faith, which admits our inability to help ourselves and rests wholly on another for salvation, confirms that our works have no power to atone for our wickedness. John Calvin writes, “If, on the part of God, it is grace alone, and if we bring nothing but faith, which strips us of all commendations, it follows that salvation does not come from us.”
Of course, faith is something that we exercise, and so some Christians think we bring this to the table when we are saved. They think we work faith up in ourselves and that all people are born with the ability to do so. Ephesians 2:8 makes this view impossible. The grammatical gender of the demonstrative this in “this is not your own doing” must refer back to the entire complex of things Paul mentions in the verse — salvation, grace, and faith. It is true that we are the ones who put our faith in Christ, but God gives us this faith and guarantees that we will exercise it unto salvation. If the Holy Spirit changes our hearts, we will not refuse the call to trust in Christ.