I believe in progressive revelation:
That the truth is revealed to humankind one part at a time, not all at once
Nice.
That it is given when people are ready for it
I assume by people, you mean God's people.
Not sure what you mean by ready for it, but if you mean, the time God determines, I would agree.
The fact that our New Testament was added to the old Hebrew Bible shows us that this has happened at least once, within the Christian tradition
How can we be sure there won’t at some point be a Third Testament?
Very good question.
I go by what is written in Hebrews 1
1 Long ago God spoke to our forefathers by means of the prophets on many occasions and in many ways.
2 Now at the end of these days he has spoken to us by means of a Son, whom he appointed heir of all things, and through whom he made the systems of things.
The scripture specifically says, "at the end of these days he has spoken to us by means of a Son, whom he appointed heir of all things".
So if God says he speaks to us by means of this son, and this son has been appointed by God, to be heir of
all things, then in line with the rest of the scriptures, I accept that we have been given the final revelation
[see footnote], before the great day of God almighty, after which, only those who find themselves in Gods new world, will know that they made the right choice.
footnote
(Revelation 1:1-3)
1 A revelation by Jesus Christ, which God gave him, to show his slaves the things that must shortly take place. And he sent his angel and presented it in signs through him
to his slave John, 2 who bore witness to the word God gave and to the witness Jesus Christ gave, yes, to all the things he saw.
3 Happy is the one who reads aloud and those who hear the words of this prophecy and who observe the things written in it, for the appointed time is near.
(Revelation 22:7) Look!
I am coming quickly. Happy is anyone observing the words of the prophecy of this scroll.”
(Revelation 22:10, 11)
10 He also tells me: “Do not seal up the words of the prophecy of this scroll, for the appointed time is near. 11 Let the one who is unrighteous continue in unrighteousness, and let the filthy one continue in his filth; but let the righteous one continue in righteousness, and let the holy one continue in holiness.
(Revelation 22:18-20)
18 “I am bearing witness to everyone who hears the words of the prophecy of this scroll: If anyone makes an addition to these things, God will add to him the plagues that are written in this scroll; 19 and if anyone takes anything away from the words of the scroll of this prophecy, God will take his portion away from the trees of life and out of the holy city, things that are written about in this scroll. 20 “The one who bears witness of these things says, ‘Yes, I am coming quickly.’” “Amen! Come, Lord Jesus.”
This is specific in saying that nothing should be added or taken away from Jesus final message before he comes to execute judgment on his enemies - those deserving death, and reward his slaves - those he gives life.
I think anyone could write such a thing, but that the question of to what extent such a thing would ever be accepted is a different issue!
Indeed I should imagine such an effort would be largely rejected amongst believers
For good reason too.
I think if any book qualified as material close to the Bible, it would give evidence of the Devil's handiwork.
The Bible describes that one as cunning, and a liar, seeking to devour as many as he can lead away from the truth.
What do people think?
Edit: I should imagine one would have to convince people it is somehow inspired by God!
How would they do that, other than by using the Bible in such a way (2 Peter 3:16) to step on.
The only way that one could go about even providing something near convincing that their book is from God, would be to disprove John's words in Revelation, which tells us everything that is to come to pass, including the raising of the dead, the complete destruction of the Devil, and death, and the judgment to life... as well as the rule over the new earth, where suffering and pain and death are no more.