The rationale that actually needs to be looked at is this: Why, after thousands of years of sending messengers and prophets preaching one thing, God suddenly changes His mind and decides that he is composed of three persons/essences/godheads i.e. a trinity, and suddenly decides to beget offspring. And, why God would present as the main crux of the belief of a faith, a concept that doesn't make sense, people don't fully understand, can't explain, and so end up saying, "Well, you don't need to understand what you believe in", or, "The Lord works in mysterious ways".
When God (God forbid) suddenly becomes inconsistent in the message He sent, alarm bells should ring.
What Allah did tell us, is the most important thing that we need to know. That there is no trinity, that He did not beget a son, and that prior to the Qur'an, people have been writing things down and then themselves or others claiming it as a scripture/from God etc, and that the Qur'an comes confirming the message of the original divinely revealed scriptures. Allah tells us the scriptures have been changed/intermingled with the words of humans, and also tells us what the original message was in His Divinely revealed scriptures, and that is what we need to know, so that we can come back to the right path.
Peace.
