Verse 21 is the benediction of the aged apostle upon the churches. The last words of the book of inspiration are a blessing. God does not permit the worship of any created being or thing in heaven or earth. He in person, or as manifest in Christ, is the only object in the universe before whom man should bow in adoration. God is a jealous God.
Idolatry is a grievous sin.
Old Israel was rejected from Jerusalem and sent into captivity for idolatry. Those who claim to be of Israel and are idolaters will be rejected from the New Jerusalem. Those who obey Him, wash their robes and make them white in the blood of the Lamb, shall have a right to the tree of life, and to pass through the gates into the city. The wicked, the idolatrous, the deceivers, those who love not the truth, shall be excluded forever. The invitation is now cordial, earnest, and universal.