How is salvation by faith alone if you have to believe?

from the old testament

• The Old Testament points to salvation by faith through examples of trust and reliance on God, not by works. Abraham was “counted righteous because of his faith” (Genesis 15:6). Trusting God, not performing works, brings right standing with Him.
• By believing God’s warning and acting in obedience, Noah was saved through faith, not by merit (Genesis 6:8–9). His faith led to action, but it proved his faith; it did not save him.
• The law pointed forward to atonement; the sacrifices themselves didn’t save, but faith in God’s provision highlighted dependence on Him. For example, the sin offering and guilt offering emphasized that atonement was God’s, not the offerer’s (Leviticus 4:20, 22, 26, 31, 35). The Passover lamb foreshadowed deliverance through God’s provision, requiring Israel to trust God’s instruction for protection (Exodus 12:7–13). Even the repeated daily burnt offerings reminded the people that continual dependence on God, not human merit, was central to fellowship with Him (Numbers 28:3–4).

from the new testament

• Faith is the conduit through which God applies salvation. However, even saving faith is a gift from God: “For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast” (Ephesians 2:8–9; cf. Philippians 1:29). Salvation is by grace, done through faith.
• Faith is “instrumental," not “causative," i.e., faith is not the cause of salvation but the instrument God uses to apply salvation. Paul refers to this instrumental sense when he says, “we have been justified by faith” (Romans 5:1a) and that "“this is not your doing; it is the gift of God,” referring to the entire package of “by grace … through faith" (Ephesians 2:8-9). It’s not that the instrument saves us, but that God uses it to apply salvation (cf. Acts 13:48).
• Why does God need to work in anyone to believe? Because by nature, we are “children of wrath” (Ephesians 2:3). Paul, quoting various Old Testament passages, wrote, “None is righteous, no, not one; no one understands; no one seeks for God. All have turned aside; together they have become worthless; no one does good, not even one” (Romans 3:10–12; cf. Romans 8:7–8). He describes unbelievers “dead in our trespasses,” requiring God to make “us alive together with Christ” (Ephesians 2:5).
• That’s why Paul says that it is “by grace you have been saved” (Ephesians 2:5c). Grace is the cause; faith is the divinely given instrument.
• What about passages like Romans 10:9–10, which reads, “if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. For with the heart one believes and is justified, and with the mouth one confesses and is saved”? Paul was making a point about those who were trying to earn salvation through works (Romans 10:5), not answering the question about faith’s relationship to salvation.
• In English, Romans 10:9–10 reads as if belief causes our salvation. However, Paul uses present-tense language to indicate that belief describes the ongoing reality of a believer’s life.
• Scripture clarifies how God accomplishes all of salvation: The Father draws and calls (John 6:37, 44, 65; Romans 8:30), and everyone He calls is saved through the faith He gives us (John 10:27-29). Jesus lived perfectly (Hebrews 4:15), and the Father applies Jesus' righteousness to those He has granted faith. Jesus said that everyone the Father gives Him will be saved because Jesus will not lose any of them (John 6:39–40). The Father and Son apply salvation through the work of the Holy Spirit, who brings regeneration (John 3:5–8; Titus 3:5) and continues to grow us to be more like Christ (Galatians 5:22–24).

implications for today

Theologians have often struggled to logically “order” the components of salvation, since several appear to happen simultaneously. God grants faith, and we respond by believing. Logically speaking, God’s gift comes first, but in real time, the two happen at the same time.

At the same time, accepting a gift is not the same as doing something to earn it. For example, if a friend thinks to buy you a present, chooses one, pays for it, wraps it, and gives it to you, your accepting it does not change anything to receive the gift. Your accepting it adds nothing to the gift itself nor the receiving of it. So it is with faith. God saw our need because of our sin, chose how He would solve the problem, provided Himself as the means to the solution, and gave Himself, dying on a cross and rising again, conquering sin and death, so that all who believe in Him can be saved. Our belief in Him does not add anything to the act of salvation; nothing we do earns it.

When Paul says that we are saved by faith and not by works (Ephesians 2:8–9), we can trust that it is true even down to the detail of belief. Ultimately, our salvation does not rest on anything we do, but on everything God has done for us.