Any self-proclamation by a text that it is the final truth is obviously not acceptable.
Christians believe that the Bible is the Only truth, which is final. I do not believe that. I believe that more truth from God has been revealed since the Bible was canonized. I believe that the verses that say not to add to the Bible mean not to add to the Bible after the Bible had been canonized. Those verses do not mean that no further truth from God could ever be revealed by God after the Bible was canonized.
Deuteronomy 4:2
Ye shall not add unto the word which I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the Lord your God which I command you.
Revelation 22:18
For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book:
The question is what then would be the test? One possibility is to rely on the spiritual perception of the sages. But this also is not workable because the sages of different religions at different times have different perceptions, so they can be in conflict.
Messengers of God (Prophets), and the religions that were established after their coming, contained ‘different’ messages because the needs of the people were different in the different ages in which those Messengers appeared. Sometimes these messages appear to be in conflict when in reality they are just a ‘new message’ that was an addition to the previous message.
In my understanding the only touchstone of the truth of a Scripture is whether it has provided succour to a large number of people. If a text actually reflects the Word of God, then people should get benefit from that word. So, on that ground alone, it is possible in my opinion, to decide whether a text is spiritually valid or not.
I agree with that, as expressed in the following passage:
“What then is the mission of the divine prophets? Their mission is the education and advancement of the world of humanity. They are the real teachers and educators, the universal instructors of mankind. If we wish to discover whether any one of these great souls or messengers was in reality a prophet of God we must investigate the facts surrounding His life and history; and the first point of our investigation will be the education He bestowed upon mankind. If He has been an educator, if He has really trained a nation or people, causing it to rise from the lowest depths of ignorance to the highest station of knowledge, then we are sure that He was a prophet. This is a plain and clear method of procedure, proof that is irrefutable. We do not need to seek after other proofs.”
Bahá’í World Faith, p. 273
whether this confirms God's existence is a separate issue which we need not get into here at this point. Thank you.
Yes, that is a separate issue.