This is what Paul confronts the Galatians with in Galatians 3:1-14. As Paul instructed them previously that it was their faith in Jesus that justified them (positionally made them right with God), it was that same faith that sanctifies them (grows them to be more like Jesus). To think that you get "in" by faith, but then have to "work" to stay in and grow is the equivalent to being a fool and allowing someone to bewitch or deceive you. It would be like starting a road trip by fueling up with gas, but halfway through believing someone who tells you that you should use root beer to fuel your car. Totally foolish! As a car is built and designed to run on gas, Christ-followers are built and designed to run on faith.
To prove his point, Paul takes the Galatians back on a journey with Abraham, who is the father of the Jewish nation. His point is that God called a pagan idolater named Abram to follow him, and it wasn't because of anything that Abram did to earn this relationship with God (Genesis 12:1-3, Joshua 24:2). Then, even after his call, Abram still wasn't the poster child of a worshipper of the One True God, for he was a liar, a coward, pimped out his wife, made bad real estate decisions, and simply didn't lead like God called him to (see Genesis 12-14). Still, God initiated a covenant with Abram and only God ratified the covenant, because he knew that Abram would be unable to (Genesis 15:5-18). The only thing that was credited as righteousness to Abram was his faith. Why is all this Abraham talk important? Because it showed the Galatians that it is clearly not keeping law that justifies or sanctifies you, as illustrated from the very beginning with Abraham; and it shows us that when God makes a promise to bless everybody (including you and me), he always keeps his promises. We are "children of Abraham" because of our faith, not our good works, achievements, or commitments.
Paul goes on to show that when we impose law or have law imposed upon ourselves, the very law we live by is the very law that will kill us because of our inability to keep it. We are, in effect, "cursed". A loose paraphrase of the Leviticus 18:5 that Paul quotes in 3:12 would be "if you live by the sword, you'll die by the sword." And mostly keeping the law, mostly keeping our commitments doesn't but it, either. James 2:10 clearly says that breaking even the smallest of the law is worthy of the punishment for breaking all of the law.
So what are we to do? How are we to live for God? Well, Paul is clear: we are to live by faith. Ok, so what does that look like? Aren't there still expectation to live up to? Yes, there are. But we don't do anything. Jesus does it for us.
Here's where the beauty of the gospel shines brightly. Jesus became a curse for us. He legally took our penalty. He took our sin, we are given his righteousness. God's justice and mercy work in complete harmony. So, we are to "live by faith." Meaning, every moment of every day is lived in surrender to Jesus. When we surrender, we give up our will to the will of another. This means that our faith in Jesus is not just what initially saves us, but also continually saves us and becomes the fuel for our everyday lives. Jesus just didn't die for our sin nature, but for our sin. So everyday becomes a rhythm of confession-repentance-reconciliation-rejoicing. Our justification is secured at the moment of salvation, and then our sanctification grows. So by daily (or hourly, or minute by minute) reminding ourselves of the gospel, we let go of all of our commitments and promises we can't keep anyway, surrender our will to Jesus, and the let the Spirit do His work in our lives. 2 Corinthians 3:18 loudly resounds this work of sanctification: we are transformed more and more into the image of Jesus NOT by our efforts, but by the work of the Spirit in us!
A life of faith is not marked by empty promises and unkept commitments that result in guilt, shame, and condemnation. A life of faith is marked by surrendering our will to Jesus, empowered by the Holy Spirit, which results in love, joy, serving, and missional living because you are just so thankful.