Answer: Holiness is so important because without holiness no one will see the Lord, (verse 14).
MORE INFORMATION AND/OR OTHER SCRIPTURE REFERENCES:
2 Timothy 2:21: "If a man cleanses himself from the latter [ignoble purposes], he will be an instrument for noble purposes, made holy, useful to the Master and prepared to do any good work."
Hebrews 2:10-11: "In bringing many sons to glory, it was fitting that God, for Whom and through Whom everything exists, should make the Author of their salvation perfect through suffering. Both the One Who makes men holy and those who are made holy are of the same family. So Jesus is not ashamed to call them brothers."
Hebrews 10:8-14: "First He said, ‘Sacrifices and offerings, burnt offerings and sin offerings You did not desire, nor were You pleased with them' (although the law required them to be made). Then He said, ‘Here I am, I have come to do Your Will.' He sets aside the first to establish the second. And by that Will, we have been made holy through the sacrifice of the body of Jesus Christ once for all. Day after day every priest stands and performs his religious duties; again and again he offers the same sacrifices, which can never take away sins. But when this Priest had offered for all time one sacrifice for sins, He sat down at the right hand of God. Since that time He waits for His enemies to be made His footstool, because by one sacrifice He has made perfect forever those who are being made holy."
1 Peter 1:15-16: "But just as He Who called you is holy, so be holy in all you do; for it is written: ‘Be holy, because I am holy.'"
1 John 3:2-3: "Dear friends, now we are children of God, and what we will be has not yet been made known. But we know that when He appears, we shall be like Him, for we shall see Him as He is. Everyone who has this hope in Him purifies himself, just as He is pure."
