I believe the answer is: salvation by grace. If anyone is to be saved, it will be through grace. Salvation has always been first and foremost a matter of grace. Without grace there is no salvation! Ephesians 2:4-8 makes this clear:
"But God, who is rich in mercy, because of His great love with which He loved us, even when we were dead in trespasses, made us alive together with Christ (by grace you have been saved), and raised us up together, and made us sit together in the heavenly places in Christ Jesus, that in the ages to come He might show the exceeding riches of His grace in His kindness toward us in Christ Jesus. For by grace you have been saved through faith, and that not of yourselves; it is the gift of God."
This passage stresses grace available through Christ, but it doesn't teach salvation by grace alone (that is a subtle but very significant point). Paul states that we are saved by grace through faith. Both grace and faith are important. But, for right now, we're focusing on the fact that without God's grace, we could not be saved. Why not? Because God doesn't owe us anything! What we deserve is eternal condemnation, because all are sinners (cf. Rom. 3:23) and "the wages of sin is death" (Rom. 6:23a). But salvation is available as a gift, which God in loving-kindness offers to mankind (cf. Rom. 6:23b).
A living, obedient faith gives us access into God's grace (cf. Rom. 5:1,2), but it is not the case that our faith earns us salvation or puts God in our debt. We cannot make up for our sins or overcome them with our own deeds of righteousness (cf. James 2:10). But we can obey the gospel of Christ, calling out to God for mercy and grace (cf. Acts 22:16). When we obey God in faith we are still "unprofitable servants" (Luke 17:10). We don't merit salvation even when we do what we're supposed to do. May we never forget that without God's grace, salvation is not possible (no matter how much faith we may have)!
On the other hand, it is important to understand that God's grace requires holy living. Some people reason that since we are saved by grace, we are free to live however we wish. This is false! Paul said in Titus 2:11ff that grace teaches us to:
As he continues, Paul explains why Jesus, in grace, gave Himself for us:
You see friends, the fact that our God is gracious should motivate us to live a holy life for Him. In other words, if we really appreciate what God has done for us and if we really understand the magnitude of the gift He has given us, we will be inspired to live righteously.
We will conclude our thoughts on this theme in our next lesson.