It would have been very easy for older religions to present strong evidence for divinity: just have some explicit, non-trivial facts in the holy book that were unknown to humankind when those books were written, like earth-moon/sun distance, speed of light, etc.
Complete absence of anything like that is a pretty strong indicator by itself...
Then the Lord God formed a man from the dust of the ground and breathed into his nostrils the breath of life, and the man became a living being.