What was the tree of knowledge of good and evil, and why was it a sin to eat its fruit?
Bruce Waltke is very helpful, on page 86 of his Genesis commentary:
‘Good and evil’ is a merism for all moral knowledge: the capacity to create a system of ethics and make moral judgements. The knowledge of god and evil represents wisdom and discernment to decide and effect ‘good’ (i.e., what advances life) and ‘evil’ (i.e., what hinders it). Unless we know everything, we only know relatively; unless we know comprehensively, we cannot know absolutely. Therefore only God in heaven, who transcends time and space, has this prerogative to know truly what is good and bad for life. Thus, the tree represents knowledge and power appropriate only to God (Genesis 3:5, 22). Human beings, by contrast, must depend upon a revelation from the only one who truly knows good and evil (Proverbs 30:1-6), but humanity’s temptation is to seize this prerogative independently from God.