God will reward our faithfulness with salvation. That seems to contradict the Protestant pillar of Sola Gratia, but it actually doesn't. First, let's look at what Jesus says, and then we'll unpack this doctrine. Jesus says to the church at Sardis after rebuking it for spiritual deadness,
Yet you have still a few names in Sardis, people who have not soiled their garments, and they will walk with me in white, for they are worthy. The one who conquers will be clothed thus in white garments, and I will never blot his name out of the book of life. I will confess his name before my Father and before His angels. He who has an ear, let Him hear what the Spirit says to the churches. (Revelation 3:4-6).
Despite the fact that most of the church at Sardis was dead, there were still a few of them who had "not soiled their garments." They hadn't given in to false teaching or rampant sinfulness. They remained faithful to Jesus. We already know that no one does this perfectly, but we also need to understand that we can do this to some degree. Don't believe the lie that we are unable to display spiritual vitality. It's required by God and made possible by God. We need to know that we must not soil our garments, and we have the ability not to. Again, it won't ever be perfect as we continue to struggle with the flesh, but our spiritual aliveness is a real thing that can and must be maintained.
For these people, Jesus says, "...they will walk with me in white." The fact that Jesus is walking now in white garments symbolizes that He is pure (Psalm 51:7), holy (Isaiah 1:18), righteous (Revelation 19:8), victorious (Revelation 6:2), and glorious (Matthew 17:2). Jesus was the first human to walk in white, and all those who continue to not soil their garments will themselves walk with Him in white.
Notice why they walk with Him in white: "...for they are worthy" (v. 4). This language can make us Protestants uneasy. None of us is worthy, we say. And that's partly true. But Jesus says here that those who do not soil their garments, those who remain faithful to Jesus, will walk with Jesus in white because they are worthy.
He goes on to say in verse 5, "The one who conquers will be clothed thus in white garments." As we've seen before in these last two chapters, those who conquer are those who persevere to the very end. Such people will be clothed in white garments. The garments of the saints on earth are clean, but not white. (Follow us symbolically here.) But those who get to heaven unstained will be given new garments, white as snow.
Furthermore, Jesus says of such people, "...and I will never blot his name out of the book of life." In other words, such a person is in the book of names of those who will live forever and ever through Jesus Christ. Their names will never be blotted out (erased by having ink blots put over their names). Such people will live forever.
Finally, He says, "I will confess his name before my Father and before His angels" (v. 6). He will declare such people His own in the heavenly celebration.
What we can't escape, even as Protestants, is that Jesus' language here is that if a person does A, then He will do B for them. It is a reward. There is a sense in which salvation is a reward for obedience. The question is, "In what sense?"
This is why it's helpful to understand the various parts of salvation. Justification is the act of God's declaring us not guilty. We didn't earn that in any way. The Spirit woke us up from our spiritual deadness and slumber, and we responded to the gospel in God-given faith. We did not work for justification.
However, we do work for glorification, in a way. We work for glorification through sanctification.
Hebrews 12:14 says, "Strive for peace with everyone, and for the holiness without which no one will see the Lord." You will not see the Lord without holiness. And this isn't imputed holiness, either, because one does not "strive" for the holiness of Christ. What we do strive for is personal holiness, and without personal holiness, we will not see God.
Now, in the end, we will see that God was the One who ultimately did it all. After Paul exhorts the Philippian Christians to work out their own salvation, he then tells them, "for it is God who works in you, both to will and to work for his good pleasure" (Philippians 2:13). If you desire to live for God, that's from God. And if you live for God, that's from God also. So, when we look back, we'll give all credit to God.
However, God working in you looks like you persevering to the end. It looks like you killing sin and remaining faithful to Jesus. Again, it's all His work ultimately, but He bears it out in your obedience and striving. So, don't neglect to obey Him and strive for the holiness without which you won't see Him. If your life is devoid of obedience and striving, then it's evidence that He is not working in you, which is evidence that you have never truly believed in Jesus.
Therefore, fight. Work. Conquer. And when you do, you will walk with Jesus in white, He will confess your name before His Father and His angels, and He will never blot your name out of the Book of Life. And you will understand that it was Him who was keeping you all along. So, be faithful by His grace, for His glory, and for your good.