I believe that justification by faith is the foundational truth of Christianity. You cannot know true rest and peace until you are convinced you can never be made right in God's eyes by your own works of righteousness.
If you don't understand the perfect righteousness of Christ that is yours by faith, you will lead a life of toil and sweat. You'll spend your days trying to please God through legalistic, hopeless attempts to establish your own righteousness. But the truth is, you'll never have any righteousness to bring to the Lord!
No doubt you are familiar with the passage in Isaiah that says all our righteousness is as filthy rags in God's sight (see Isaiah 64:6). This does not mean God despises our good works — not at all. We should do good works, but if you think your good works merit your salvation, that they allow you to stand holy before God, then they are nothing but filthy rags!
You may feel good because of the good works you do and even enjoy a moment of victory whenever you resist temptation. You feel righteous, that God's favor is on you.
The next day, however, you fail. You fall back into a sin and suddenly you lose all your joy. You think the Lord is angry with you and wonder if you have lost your salvation.
It is a roller-coaster ride of emotional highs and lows — of up-and-down, hot-and-cold, sin-and-confess — according to how good or bad you think you have been on any given day. It's a life of misery because you are trying to please God in your flesh!
Beloved, no righteousness of the flesh will ever stand before God. Even the best people among us, the most moral, godly saints, have fallen short of God's glory. None of us can ever be accepted in the Father's eyes by our good works. We are accepted by Him only as we are in Christ!
"For ye are all one in Christ Jesus" (Galatians 3:28). When we turn to Jesus with saving, self-emptying faith, we become one in Christ. Being "in Christ" means God credits Jesus' righteousness to us. All our sins are washed away because of His work, not ours!