by Matt Slick
Salvation is by faith alone in Christ alone because all of our righteous deeds are tainted by sin (Isaiah 64:6) and are not good enough for us to merit salvation. Therefore, because there is nothing we can do to obtain it, there's only one thing left for us to do, trust in what God has done in the person and work of Jesus.
Salvation is being delivered, being "saved", from the righteous judgment of God. All of us have sinned (Romans 3:23) and are under a legal judgment from God. Jesus equated sin with a legal debt when he said in the prayer that begins "Our Father who is in heaven," when he said in Luke 11:4, "forgive us our sins" and in the parallel version of Matthew 6:12 he said, "forgive us our debts." Jesus equates sin with debt. In fact, John the apostle says that sin is breaking the law of God. So when we sin we have broken God's law and have incurred a legal debt. The payment, or the judgment against us for breaking the law, is death (Romans 6:23) and separation from God (Isaiah 59:2). Therefore, in order to be saved from God's righteous judgment we need to be saved by what God has provided for us in the sacrifice of Christ on the cross. That is the gospel (1 Corinthians 15:1-4).
God is the standard of righteousness, not us. God says to be holy because he is only (1 Peter 1:16). This means that the standard is the character of God. Holiness is the quality of God that is synonymous with his perfection, his sinlessness, and his inability to sin. He is perfect and righteous in every thought and every deed. His holiness is a quality of his essence a powerful and pervasive presence of incredible purity. This is the standard that God wants for us because it reflects his greatness and perfection. But because we fall short in our sins, we are not holy. Therefore, because everything we do is touched by sin, we are not able to accomplish what is necessary in order to be saved from the righteous judgment of God. God knows this and so he provided for us a means of salvation. That means is through the incarnation of Jesus who is God in flesh (John 1:1, 14; Colossians 2:9), who never sinned (1 Peter 2:22), and who bore our sin in his body and the cross (1 Peter 2:24). remember that sin is a legal debt. Our legal debts were transferred to Christ and canceled on the cross (Colossians 2:14). Jesus did it all. Therefore, all we have left is our faith in what God does. This is why the Bible says we are justified by faith (Romans 5:1) apart from the works of the law (Romans 3:28).
So, faith alone in Christ alone is the only way to be saved because that is the only way to put complete and total trust in the work of God and not in our ability to be good enough to achieve or maintain our salvation.