How can you be confident that you will be made whole?
Know Who God is.
1 Thessalonians 5:24 refers to God as "the God of peace," or literally translated "the God of THE peace." He is the God of shalom – of wholeness. You can find "pieces of peace" in lots of places (money, friends, food, etc). But real peace can only come from God.
How does knowing this give you confidence that God is still at work in the heart of those who call Him "Father?" Well, sin is war against God. It is the shattering of shalom. If God is THE God of THE peace, then we can know that He has brought wholeness and is bringing wholeness to His children. Through the Prince of Peace, He makes shalom possible.
Know Where God works.
That verse goes on to say that He is sanctifying your whole spirit, soul and body. Sanctification is not a process that happens in one corner of your life. Maybe you've wanted to tell God, "Welcome to my heart! Make Yourself at home in the kitchen and living room, but stay out of my bedroom. Don't open that closet." But it doesn't work that way. There are no places where you can put up a "no sanctification here" sign.
So, we know that God wants to make you wholly whole. God loves you so much that He's not willing to leave any part of you untouched by His love. He's doing a top to bottom work that affects your head, heart and hands equally.
Know When God Finishes.
".. be kept blameless at the coming of the Lord Jesus Christ." This is a past, present and future thing. It's a past work because we have been made blameless before God because of Jesus. That work is done. It's a present work because God is preserving us until the coming of Christ. And it's a future work because one day we will be made whole forever in the presence of God!
So, how can you know that God isn't done sanctifying you? Because He Who has begun a good work in you will finish it (Phil 1:6).
Know What God does.
"He Who calls you is faithful; He will surely do it." The Caller is the Doer. This is a simple idea, but it can be so hard to believe. Maybe you feel like God has been working on you for years, and He must be ready to give up. You feel like He has given you a fair shake and is ready to call it quits.
But that isn't our God. He is both the Caller and the Doer. If you are in Jesus, then you have been called. And if you have been called, then God's Word promises that our Father is good and faithful. He hasn't finished working in you yet. He is still making you whole. He is still making you like Jesus.