Hello
@CoreIssue,
No, they did not have the written record that we have in God's Word, but we do, and we are called upon to believe what He says within it, and the record given concerning Abel is sufficient for us to know, that his offering was made on the ground of faith. He had believed God and acted upon it, that is faith obedience. For Cain to have had his offering rejected, is sufficient proof that he had not so acted.
So, no I do not believe that I am assuming anything, just believing what God has said, in His Word. Today we have God's Word written, but in the days of Cain and Abel, God revealed His will to them by the spoken word, and that is what they were called upon to believe and act upon.
Thank you.
In Christ Jesus
Chris