"It is by grace you have been saved, through faith — and this is not from yourselves, it is the gift of God." — Ephesians 2:8
The word faith can have several meanings. Sometimes it means simple assent to a fact or an idea as true. Sometimes it refers to a body of religious teaching. In the Bible it can sometimes refer to a special amount of faith, as in "a faith that can move mountains" (1 Corinthians 13:2). But the most common use of faith refers to an implicit trust or reliance on something or someone.
For Christians, that sense of trust or reliance points to saving faith in Jesus Christ. As Ephesians 2:8 points out, we are saved by grace "through faith." In contrast to the church teaching of his time, Martin Luther rediscovered that salvation is by grace and is received through faith. Grace wrapped in faith is the gift of God.
Instead of seeing faith as something required in order to receive grace, Luther found that faith is a channel of God's grace. In other words, even the faith we need in order to acknowledge Jesus as Lord comes graciously from God through the power of the Holy Spirit. If it were any other way, we could boast: "I found Jesus!"
Paul emphasizes that no boasting is allowed. God finds us. Even the faith to accept Jesus comes through grace alone. Indeed, grace alone and faith alone are like two sides of the same coin.