In the third and fourth chapters of Romans, Paul seems to be pretty hung up on the idea of faith…
“…the righteousness of God through faith in Jesus Christ for all who believe.” (3:22a)
“For we hold that one is justified by faith apart form works of the law.” (3:28)
“…his faith is counted as righteousness.” (4:5b)
“For the promise to Abraham and his offspring that he would be heir of the world did not come through the law but through the righteousness of faith.” (4:13)
“That is why it depends on faith, in order that the promise may rest on grace…” (4:16a)
I don’t know if you noticed the theme, but it’s faith. Why must we be saved by faith? Well Paul touches on that in the last quote from Romans 4:16, “in order that the promise may rest on grace.” By faith we are trusting that God will do every single thing required for our salvation. None of it depends on us; it all rests on His grace. Why must it rest on grace, you ask? Great question, and Paul digs into this a little deeper in chapter 8.
“For God has done what the law, weakened by the flesh, could not do. By sending his own Son in the likeness of sinful flesh, he condemned sin in the flesh, in order that the righteous requirement of the law might be fulfilled in us, who walk not according to the flesh but according to the Spirit.” (Romans 8:3-4 ESV)
We must have faith that God will save because we cannot save ourselves. The law can show us where we have failed, but cannot help us change that. Imagine, if you will a married couple, we’ll call them Tom and Jane. If Tom lies to Jane one time, and then tells her the truth one time, should she trust him? No! (Or as Paul would say, “By no means!”) Tom has only told the truth 50% of the time. He is a liar and not to be trusted. What if Tom told one lie and then told the truth 10 times? Better but still very untrustworthy. 1 out of 10, still a liar. What if he told the truth 100 times? 1000 times? 1,000,000 times? Well, a million is a lot. We’d probably trust him then. But does the million times change the fact that he lied? No. By lying one time, he is by definition a liar, and there is nothing he can do to change that. He may become more trustworthy, but he cannot change the lie.
We are by definition sinners. We have fallen short of the goal of perfection that God has set. And we now, by faith, believe that God will act on our behalf. We trust that He will do the one thing we cannot; He will change who we are. He will erase the lie. He will place the lie on His Son Jesus at the cross and condemn it. God makes us righteous, not because of what we have done, but because of what He has done. And we believe.