What does 1 Samuel 15:23 mean?
Saul has excused his disobedience to God's command by blaming the people. He claims "they" retained the spoil and said they were going to sacrifice it to the Lord (1 Samuel 15:2–3, 15, 20–21). Samuel explained to Saul that God desires obedience, not sacrifice (1 Samuel 15:22). We can't make better choices than God, and we can't bribe Him to excuse resisting Him. Samuel goes on to highlight the seriousness of Saul's sin.It's not clear if Saul and the people originally intended to sacrifice the spoils at Gilgal, or if they first planned to keep the animals for themselves. Given that Saul set up a monument to himself (1 Samuel 15:12) and that the command from God about total destruction had been so clear (1 Samuel 15:3), the intent to use the animals for worship is extremely questionable. Even if it had been honest, Saul's disobedience was rebellion against the direct command of the Lord. Samuel now says that rebellion is just as bad in God's eyes as the sin of divination.
Divination is an attempt to persuade spiritual beings to reveal otherwise unknowable truths by offering them gifts. This often involved meals, money or specific rituals involving animal parts. The same basic motive is behind mediums and necromancy: to learn what God has chosen not to reveal. Divination was clearly understood to be evil, and God forbid the Israelites from having anything to do with it (Deuteronomy 18:10; Ezekiel 13:23). Even today, those practices indicate a rejection of God's authority, and naivety about spiritual dangers (Ephesians 6:12).
Even so, Samuel's statement that disobeying God is as evil as practicing divination is almost shocking. He adds that presumption or arrogance is as evil as participating in idol worship. Saul had arrogantly presumed he could manipulate the Lord into giving His favor if he offered enough sacrifices. Or, that he could improve on the plans which God made, as if the Lord were just another house spirit to be appeased with rituals.
Finally, Samuel declares the Lord's decision to reject Saul as king. In response to earlier disobedience, the Lord had already declared that Saul's family line would not occupy the throne of Israel (1 Samuel 13:14). Now God rejects Saul himself. Saul had rejected God's word, and thus God would reject him as king. Saul's disobedience was egregious, and the consequences were severe.