In the beginning, after God had created Adam and Eve, He loved to fellowship with them while they walked together in the Garden of Eden. Adam and Eve did not know about sin and evil because they were innocent and pure. God gave Adam and Eve only one commandment to obey. They were not to eat of the tree of the knowledge of good and evil. From the beginning, God has given mankind the power of choice, and He has never revoked it.
Satan, a powerful fallen angel, deceived Eve by tempting her to eat of the fruit from the forbidden tree. He told her she could disobey God without any consequences, and that eating the fruit would give her godlike knowledge. Then Eve chose to eat and offered the fruit to her husband, and he made the same terrible mistake.