How do we know that only holy people will enter heaven?

Revelation 21:27 emphasizes the purity required to enter heaven, stipulating that nothing impure or sinful will be allowed. This is consistent throughout the Bible, where holiness is a prerequisite for seeing God, as stated in Hebrews 12:14. Since all have sinned and fall short of the glory of God (Romans 3:23), the necessity for a divine provision for holiness arises. That provision is fulfilled in Jesus Christ, who makes His people holy through His atoning sacrifice, allowing them to enter into God's presence.