What does 1 Samuel 3:18 mean?

Samuel has been visited in the night by the Lord God of Israel, with a prophecy of judgment against his own guardian and mentor, Eli the priest (1 Samuel 3:10–14). Eli knows Samuel has received a prophecy. He warns the boy of his solemn responsibility to report the entire message without hiding any part of it. Now Samuel does exactly that, telling Eli about God's inescapable punishment against him and his household for the blasphemy of his sons which he failed to stop. Samuel must have wondered how Eli would respond. He was reluctant, at first, to pass along the message (1 Samuel 3:15).

For all his moral and spiritual failures, Eli is humble and resigned, rather than argumentative. And yet, he expresses neither repentance nor sorrow. It's almost as if the old priest shrugs and says, "Oh, well, what are you going to do?" His literal words reflect that exact sort of indifference mixed with fatalism: let God do what God will do. Eli recognizes he cannot stop what is coming and decides not to engage emotionally about it.

Why did the Lord give Samuel this same prophecy that had already been delivered to Eli by another prophet (1 Samuel 2:27–26)? It may have been to confirm for Eli that it was true. It also provided confirmation for both Eli and Samuel that the message from the Lord to Samuel was genuine. This reinforced that God had really initiated young Samuel as a prophet in Israel. Given the nature of the message, it also demonstrated that Samuel's loyalty was to the Lord: that he would speak truth, even in difficult circumstances. In hearing and giving the message, Samuel also saw the holiness of God and the reality of judgment for those who would scorn His ways. He knew not to follow the example of Eli's sons, and he knew that a change would be coming at the temple. His trust must be in God Himself, not in his mentor.