What does 1 Samuel 3:17 mean?Eli has been Samuel's guardian and mentor since the boy was a very young age (1 Samuel 1:24–28). His blessing on Hannah had been used by the Lord, in part, to grant her request for a son (1 Samuel 1:17). He had likely raised Samuel as a son, at least in some sense, training the boy in the ways of the temple (1 Samuel 2:11, 18–20, 26; 3:1). Then, on the previous night, Eli had realized that the Lord God of Israel was calling out to Samuel in a voice the boy could hear (1 Samuel 3:4–9). This meant the boy was entering the realm of prophets.
True prophets were called on to deliver God's messages in their entirety and without changes (Deuteronomy 18:20–22). So, Eli uses language closely tied to prophetic oaths. We see this type of oath language in other places, such as when Ruth swore her allegiance to Naomi (Ruth 1:17) and when Jonathan promised to warn David if Saul wanted to kill him (1 Samuel 20:12–13). We also see it used to confirm a threat, such as when Abner changed his loyalty from Saul's line to David (2 Samuel 3:9–11) or when Jezebel threatened to kill Elijah (1 Kings 19:2). Here, it seems Eli is using this as a mentoring opportunity with Samuel. He tells Samuel to deliver the entire message he received from the Lord and not to hide any of it. Assuming it to be a message of judgment, which prophecies often were, Eli warns Samuel that the Lord will do to him at least what's described, plus whatever parts of the judgment prophecy Samuel leaves out in the telling.
This sounds harsh, at first, but Eli is instructing Samuel on the full responsibility of a prophet of the Lord. Those entrusted with God's messages must not under any circumstances fail to deliver God's words to the intended audience (Jeremiah 1:4–8; Ezekiel 33:7–9; Acts 20:26–27). Samuel needed to learn this.
Eli probably didn't know the judgment prophecy was about him and his family. Then again, he might has suspected it; Eli had been given a similar and more detailed prophecy previously (1 Samuel 2:27–36). In any case, he receives Samuel's words without defense (1 Samuel 3:18).