5:46 he wrote of me. Moses was considered by the Jews to have been the author of all the Pentateuch, the books of the Law. The modern speculations of higher criticism, which chop up the Pentateuch into segments supposedly written by various authors, all of whom lived long after Moses’ time, were unheard of among these ancient scholars (as well as Christ Himself!) who lived much closer to Moses’ time and had access to much more data on the subject than modern scholars. These writings include the book of Genesis, which actually was written by eye-witnesses, then handed down from patriarch to patriarch until it finally was incorporated by Moses with his four other books. Thus, even Genesis was (and is) considered to be one of the books of Moses, even though its tablets were only documented and edited by him.
5:47 believe not his writings. It is foolish to claim (though many modern Christians do) to believe the New Testament without also believing the Old Testament, including Genesis. Christ Himself accepted the Pentateuch as literal and inspired history. Thus, to reject the Genesis accounts of Creation and the Flood, for example, is tantamount to rejecting Christ as the omniscient Son of God.