Holiness is an extremely important word to understand. Without holiness no one will see God (Hebrews 12:14). If that is true, the obvious questions are as follows.
Are you holy? How do you know if you are or are not holy? Are you at peace with God?
To start with, holiness is not an achievement earned by human effort rather it is a gift received by faith in God’s holiness. Holiness is how God sees us if we are free of sin.
If anyone has sin, they cannot enter heaven. That is a real predicament if we are counting on being holy enough to deserve God’s acceptance into heaven. No one but Christ is holy. Only Christ has lived a sin free life so we can be holy only if we believe that He took away our sin on the cross. He is our holiness. What we cannot possibly do for ourselves, He did for us.
It is because of him that you are in Christ Jesus, who has become for us wisdom from God—that is, our righteousness, holiness and redemption. 1 Corinthians 1:30
We do not need to worry about ever being rejected by God because He has taken away all our condemnation according to Romans 8:1. There is now no condemnation for those who are in Christ Jesus. Absolutely positively no sin whatsoever is allowed in heaven. All of us know we sin and we truly do not deserve to be in heaven. But God in His graciousness has opened a new way for us to change from sinner to saint. Christ’s sacrifice on the cross was the payment for all sins past, present and future. He died that we might live a new life no longer guilty because He has taken all the punishment for sin upon Himself (Isaiah 53:5). So what must we do to be saved, to be holy? BELIEVE in the Lord Jesus Christ and you shall be saved (Acts 16:31). It’s all about believing or not believing.
For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. For God did not send his Son into the world to condemn the world, but to save the world through him. Whoever believes in him is not condemned, but whoever does not believe stands condemned already because they have not believed in the name of God’s one and only Son. John 3:16-18
More on holiness at https://www.gotquestions.org/holiness-Bible.html