Why does God judge His people if they have already repented from their sins and the blood of Jesus has washed their sin away?

This is a very good question. If people are saved, why must they appear before the judgment? The Bible says that everyone will stand before the judgment seat of Christ; that must include professed believers (Romans 14:10). But those who have sincerely accepted Christ need not fear the judgment, because His blood covers them. They are judged as righteous by virtue of His righteousness, but they’re still judged.

In 1 Peter 4:17, God’s Word teaches, “For the time is come that judgment must begin at the house of God.” There are several judgments in the Bible, and one of these, called the pre-advent judgment, is happening right now. This is the judgment concerning God’s church. It is the same kind of judgment spoken of in Ezekiel 9:6 in which the Lord declares that His judgment will begin at His sanctuary. Revelation 22:12 says, “And, behold, I come quickly; and my reward is with me, to give every man according as his work shall be” (emphasis added). You can also read about this judgment in Daniel 7:10, where “the judgment was set, and the books were opened.”

Christians are being judged through the things written in these books, before the judgment throne of God. But we are not actually standing present for this judgment going on in heaven because we are here on earth, nor are those deceased Christians (who are still in their graves).

So why the pre-advent judgment—the one that occurs before Christ comes with His reward? It’s because many people call themselves Christians, but really aren’t. Jesus said, “Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven. Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? And then will I profess unto them, I never knew you: depart from me, ye that work iniquity” (Matthew 7:21–23). Within the church, hypocrites abound—we would all agree with that. Judas is one example of someone who professed faith and performed miracles but did not do the will of the Father. When Jesus comes, the angels—those looking into the books—will know who they are coming to take to heaven.

We must always remember that the judgment is not for the Lord’s benefit; He does not need to discover our true status because He already knows all things. But there are countless other intelligent beings intently interested in what has transpired here on earth. This pre-advent judgment helps to demonstrate that those who have professed Christ are genuinely transformed. (See Romans 12:2 for the Bible’s definition of transformation.)

We must be careful not to interpret this judgment to mean that if you accept Jesus and are baptized but then sin again, you are forever lost because of those sins. You still must repent of your sins, but you are on a new walk. When you’re baptized, you are justified. You have come to the Lord, you’ve consecrated your life to Him, and your old sins are washed away.

After you leave your personal Egypt, you begin a wilderness journey on your way to the Promised Land—a process called sanctification. In this part of your spiritual experience, you learn to follow Jesus and be more like Him. That’s a process called a walk—like how a baby learns to walk.
Of course, if a baby still can’t walk when he or she turns five, there’s a problem. So I think we must be showing progress and every day become more like Christ.

We might also consider the purpose of this pre-advent judgment as a loving action on the part of our heavenly Father. Perhaps one reason He begins with those who already had the opportunity to know the truth is to delay judgment of those who don’t know so that they have as much time as possible to find out and accept His truths.