Revelation 22:14-17
Some pastors and theologians use Rev 21:8 and 22:15 to try and prove that all "true" Christians persevere in the faith.
Verse 14 of Revelation 22 reads, "Blessed are those who do His commandments, that they may have the right to the tree of life, and may enter through the gates into the city."
Two things are promised to the one who obeys the Lord as a characteristic pattern of life (no one obeys perfectly, cf. 1 John 1:8, 10): (1) the right to the tree of life and (2) the right to enter into the New Jerusalem through its gates. At first reading these may seem to be things which are true of all believers. However, that is not the case.
What is "the right to the tree of life"? It is the right to eat its fruits. Compare Rev 2:7, "To him who overcomes I will give to eat from the tree of life."
The tree is called "the tree of life" for a reason. It will evidently grant to the believer who eats of it a special abundance of life.
It is true, of course, that the tree of life was in the Garden of Eden as well. However, its fruit would not have had the same effect on fallen people with ungloried bodies as it will have in eternity on saints with glorified bodies. The tree of life will only grant abundant life to those with glorified bodies. According to Gen 3:22 the reason God removed Adam from the garden was "lest he put out his hand and take also of the tree of life and live forever." Had he eaten that fruit Adam would have lived forever in a state of separation from God (spiritual death). of course, since the tree of life was never intended for that terrible purpose, God took it away from man until the eternal kingdom.
Most likely all believers will be going in and out of the New Jerusalem from time to time. Some believers in eternity will have their primary dwelling in the New Jerusalem. Surely those people will sometimes venture outside its walls, visit the rest of the new earth, and return. And, many saints will not live in the New Jerusalem! According to Rev 21:24 the new earth will contain many nations and the kings of those nations will travel to the New Jerusalem to take tribute to the King of kings. It is likely that all who live in these nations will make trips to the New Jerusalem.
For another thing, the gates will not be the only way by which someone could enter the city. For example, people might travel by air, flying over the walls. Or, they might come in on a subway, going under the walls. It is even conceivable in light of John 20 that people might travel right through the walls!
Finally, we know from the OT that the gates of ancient cities were places of honor. The respected elders of the community were allowed to sit in the gates and it was from there that they rendered judgments in legal matters (cf. Gen 19:1; 22:17; Deut 22:15; 25:7; Ruth 4:1-12).
Verse 15 is describing the condition of people at that time, not their experience in this life. No believer with a glorified body could be described as being a dog, a sorcerer, a sexually immoral person, a murderer, an idolater, or a liar. While those things were true of giants of the faith like David (2 Samuel) and Solomon (1 Kings 11) in their experiences prior to death, they could never be true of saints with glorified bodies.