Does Christianity claim to be the last true religion that will ever exist, or does it admit the possibility of being supplanted by a new revelation, like Judaism was with Christianity?
Christianity1 doesn't make claims about itself but rather makes claims about Jesus. And those claims are based on the Bible, which records Jesus' own words. So what does Jesus say about being supplanted by a new revelation? He was asked exactly this question - here is His answer:
When the men came to Jesus, they said, ‘John the Baptist sent us to you to ask, “Are you the one who is to come, or should we expect someone else?”’
At that very time Jesus cured many who had diseases, illnesses and evil spirits, and gave sight to many who were blind. So he replied to the messengers, ‘Go back and report to John what you have seen and heard: the blind receive sight, the lame walk, those who have leprosy are cleansed, the deaf hear, the dead are raised, and the good news is proclaimed to the poor. Blessed is anyone who does not stumble on account of me.’
Furthermore, in another place He specifically says that there is no other way to God than through Him:
Jesus answered, ‘I am the way and the truth and the life. No one comes to the Father except through me.
1 And here you'd have to specify which flavour of Christianity you're talking about.
2 My paraphrase.
Christianity is based on the New Testament. What ever the New Testament says, that is what Christianity believes.
According to the NT, Jesus is the promised Messiah from the Old Testament, which is the scripture for the Jews. Basically, without the Jewish scripture, there is no New Testament, which means no Christianity. The New Testament claims that Jesus is the promised Messiah whom the Jews were waiting, even today. Which means that Christianity is the completion or the ultimate stage of Judaism.
Christianity itself never claims to be a new religion. Christians believe that God is a Trinity, three persons in one, known as - God the Father, God the Son and God the Holy Spirit. Since Jesus is God the Son, Christianity doesn't produce any new Deity. It's the same God from the Old Testament. Christianity claims that there are many scriptural proves in the Jewish scripture which refers to the Trinity, and also the Messiah as the Son of God who would be flogged and crucified for our sins.
In the past God spoke to our ancestors through the prophets at many times and in various ways, but in these last days he has spoken to us by his Son, whom he appointed heir of all things, and through whom also he made the universe. The Son is the radiance of God’s glory and the exact representation of his being, sustaining all things by his powerful word. (Hebrews 1:1-3, NIV)
Hebrews is a long discussion about how Jesus is supreme in every way: compared to angels, to Moses, to the high priests etc. It begins though by noting that Jesus is supreme way God has revealed himself. As Jesus said, "Anyone who has seen me has seen the Father." (John 14:9)
So Jesus is the big definitive revelation from God.
Does that mean that God has not given further revelations after the New Testament? There is no consensus on that question, and the debate between continuationism and cessationism is still ongoing.
Jesus' claims about Himself, such as there is no way to God but through Him (John 14:6) expressly reject any newer revelation that will supplant faith in Jesus Christ.
The book of Revelation shows us what will happen for a good while to come, and it shows Jesus, the Lamb of God, on the throne at the end (Rev. 21:22).
According to 1 Cor. 15:20-28 we find that the God of the Bible will endure forever; there will be no other gods from new revelations to replace Him, and Jesus is the agent through which God will draw all people to Himself.
The Bible (Old and New testaments) presents a progressive revelation of God's plan for man's salvation. Some truths are presented as eternal and unchanging, so every later revelation adds to the former without rejecting those that came before. Conflict surrounds those truths that were specifically intended to be for one dispensation but not a later one, such as animal sacrifice and certain laws intended for Israel but not humanity in general, or not for the time after Jesus' resurrection.
Some important verses:
Moses warns the people to expect another prophet (believed by Christians to be Jesus) in Deuteronomy 18:
15 The Lord your God will raise up for you a prophet like me from among you, from your fellow Israelites. You must listen to him.
Jesus taught the continuity of what came before in Matthew 5:
17 “Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. 18 For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished.
Jesus prepares his disciples to welcome and obey the Holy Spirit going forward in John 16:
12 “I have much more to say to you, more than you can now bear. 13 But when he, the Spirit of truth, comes, he will guide you into all the truth. He will not speak on his own; he will speak only what he hears, and he will tell you what is yet to come. 14 He will glorify me because it is from me that he will receive what he will make known to you. 15 All that belongs to the Father is mine. That is why I said the Spirit will receive from me what he will make known to you.”
The history of the Christian Church since the day of Pentecost has been an attempt to listen to the Holy Spirit and learn from that spirit.
To a certain degree, Scripture supplies a certain sense of finality of the revealed truths of God in Revelation 22:
18 I warn everyone who hears the words of the prophecy of this scroll: If anyone adds anything to them, God will add to that person the plagues described in this scroll. 19 And if anyone takes words away from this scroll of prophecy, God will take away from that person any share in the tree of life and in the Holy City, which are described in this scroll.
These words in Revelation stand in contrast against the many promises of future teachings and the arrival of a messiah that were predicted by the earlier prophets, like Isaiah and Jeremiah. They merely echo what Jesus said on the cross in John 19:
30 When he had received the drink, Jesus said, “It is finished.” With that, he bowed his head and gave up his spirit.
Ever since Eve was promised in Genesis that her seed would crush the serpent, mankind had been looking forward for its hope. We no longer need to look forward.
It IS finished.
When you read Mathew 16:18 and go up a few verses the rock that Jesus is referring to is the statement by Peter “You are the Christ, the Son of the living God.”
Matthew 16:18 18 And I also say to you that you are Peter, and on this rock I will build My church, and the gates of Hades shall not prevail against it.
Will it get replaced? - In the above verse the second part is your answer.
John 14:6 6 Jesus said to him, “I am the way, the truth, and the life. No one comes to the Father except through Me.