I don’t understand about the spirit world, and I don’t believe it is wise to delve into what God forbade His Old Testament people, so I have no insight about the ability of the medium to call Samuel.
Samuel told Saul that God had not only departed from him, but had become his adversary. Saul might not have minded at the time that God had departed from him, but he minded desperately now. He was a mess. Understand that this is what the outcome of sin and departure from God looks like: ultimately it leaves the forsaken a mess.
Imagine what a relief Saul’s death was for David. He had waited for God to take vengeance on Saul, and now could take the kingdom with a clean conscience. Isn’t that so much more satisfying than taking matters into his own hands? He had waited a long time to be relieved of the awful burdens of living in exile, lacking any safe place; and now he was not only free, he was king! No wonder Psalm 18 is such a happy psalm.
Psalm 18 doesn’t exaggerate the desperation of David’s predicament while Saul was alive. If it wasn’t Saul chasing him, it was the Philistine lords lurking to entrap him. God didn’t really shake the earth and smoke didn’t really come out of His nostrils when David cried to Him; this is David’s way of relating how wonderful he saw God’s response to his cry. Have you ever seen God respond to your cry in wonderful fashion? If not, I would suggest that you haven’t cried to Him in desperation, or else you haven’t cried to Him from a clean conscience, or you haven’t looked back with thankfulness and praise for what He has done. Is there an event in your own life for which you can offer your own psalm of praise today? I believe that God wants to be a hero to each one of us as He was to David. I believe that He has done heroic deeds on behalf of each one of us, if we will only make the effort to contemplate the matter prayerfully and acknowledge them joyfully as David did here.
Note in verses 20 to 24 the righteousness for which God had rewarded David. Did he leave any room for compromise in his description? If this is the kind of righteousness for which God rewarded David, would righteousness of a lower standard (is there such a thing?) receive such reward from God?
Perhaps one of the reasons David was a man after God’s own heart is that he relied on God so much. Look in verses 30 through 36 at David’s account of what God did for him. Or perhaps David relied on God so because he was a man after God’s own heart. In any case, to know God is to be like Him, and to be like Him, we’re going to have to rely on His help. If David knew God as these verses describe, might we know Him the same way? Only if we seek to do so. Seeking will certainly involve asking God for such a privilege and placing ourselves in the place to receive God’s graces, as Richard Foster describes it. But what wonderful rewards it brings! If you haven’t experienced reward in seeking God, please don’t give up seeking. As I heard in a powerful sermon this past Sunday, if you have sought God but aren’t satisfied in Him, you haven’t reached the end of the story.