Q: Is it true that just because hinduism is the oldest religion and possess the oldest scriptures (rig vedha) in the world, it has to be the the ultimate truth to know God and God is revealing himself to us through this scriptures?
A: There are several issues within the question you have asked. One is “What is the oldest scripture text in the world?” Within this question there are several hidden questions. By oldest, do we mean an actual copy of a text (extant) that has an age of great antiquity, or do we mean that by best guess and scholarship we think the original was written about a certain time? Or? that it was originally produced about a certain time.
The problem with the claim that Hinduism has the oldest scriptures is that it isn’t true in either sense (extant texts or surmised original writing). The best estimates of the age of the oldest Hindu text (the Rig Veda Samhitas) is that they were original to about 1500 BC. The reason that it is still supposedly held to be the oldest texts is that some Hindus attach very old dates to the Vedas (3500 BC and even 6000 BC). For this reason, many Hindus assert that it has the oldest scriptures. The oldest extant texts for the Vedas are very uncertain. The writing down of them was probably after 300 BC (Sacred Texts). The oldest extant text of the Vedas? I have not been able to get a clear answer to this. Many who claim an old date for an extant text clearly do not understand the meaning of the word “extant” (still in existence) and are actually referring to when they were written or when they were composed orally. I have assumed from the resounding silence and lack of demonstration of an actual old copy that the oldest extant copy is rather recent.
The winner of the oldest texts also has another wrinkle to it. Some claim that the Pyramid texts, inscriptions on the walls of some of the pyramids of Egypt, are the oldest scriptures in the world. They are dated about 2350 BC. They are certainly among the oldest extant writings we have. One might debate, however, if they are really scriptures in the sense of writings intended for public consumption or use, or if they are really inscriptions on walls of a pyramid inaccessible to people.
There are a number of competitors to the oldest scriptures: The Torah or the Tanakh would be two of them. The oldest extant texts have numerous other winners.
Your main question though is whether having the oldest scriptures means that the ultimate truth of God is revealed through these scriptures. Good question.
I think from the way you have worded your question, you have even hinted at in your question that we cannot say this is any proof of being the most reliable or ultimate truth on God. Certainly there were prior scriptures orally transmitted that we do not have that were more ancient, so that even if “being the oldest means being the best” is true, there must have existed competing ideas on God that were older. Yet, certainly one could make a case that being the oldest does not necessarily make for being the most reliable and true. I suppose one could even make a case that “newer is better.” One could certainly make a case that the scripture that corresponds most accurately with the world as we know it would be the best, or the one that is internally most consistent in the best, etc. While I do think there may be some benefits to an ancient age, one cannot logically hold to the idea that the oldest definitively makes for the ultimate or best knowledge of truth on God.
The claim of Hinduism to be the oldest religion is also very debatable. The easy answer to the question of, “Which is the oldest religion?”, is “Whatever religion is true.” For example, if the Bible is true, then it would be the oldest religion for it tells of the beginning of the creation of the world and the universe itself. If it is not true, then the description of how the world was made it not true and the events would not be true as well. Therefore the religion would not be as old as it claims to be. So though this is an easy answer to give, it is not an easy answer to know.