You make a good point here, with which I agree. Many people worldwide believe both the Old and New Testaments in the Bible are Gospel meaning a collection of infallible truths.
The folklore in the Old and New Testaments have been told orally and in written form for millenniums. First in Hebrew, then Greek and later in other languages. Who knows what was 'lost in translation'. Both written and more-so oral communications change because much of what is conveyed is someone's interpretation and is frequently embellished. Think of it as being like the telephone game, only this has been played over thousands of years and multitudes of people.
"Gospel" means "good news". The Gospel of Jesus of Nazareth is considered to be the completion of the Promise of God in the Old Testament.
It is not as though attempts to change and/or add too, the Gospel and the New Testament of Jesus of Nazareth have not been made because men being men, will seek to control others and not for the better. Look no further than to the Catholic Church and their man-influenced false doctrine, e.g., the eternal virginity of Mary the mother of Jesus of Nazareth. The same holds true for the so-called "Latter Day Saints", i.e., Mormons, i.e. MORONS. Joseph Smith was a fucking pervert bent on having sex with as many women as he could.
The message of the Nazarene is simple but as is often the case, many among mankind loathe that which is good and gravitates to the vile. It is as my own mother taught me, her words, decades old, still ring true:
Mankind desires immortality but settles for immorality. Don't believe me? Just look around you. Or in a mirror.
