Poole on 1 Samuel 3:20: Samuel, a Prophet in Israel (Part 2)
Verse 20: And all Israel (Judg. 20:1) from Dan even to Beer-sheba knew that Samuel was established (or, faithful) to be a prophet of the LORD.
[From Dan to Beer-sheba] That is, throughout the whole land; which is here described by its length; that is, from its northern border (which was Dan) and its southern border (which was Beer-sheba) (Piscator, Drusius).
From Dan even to Beer-sheba; through the whole land, from the northern bound, Dan, to the southern, Beer-sheba; which was the whole length and largest extent of the land. See Judges 20:1, 2; 2 Samuel 17:11.
[That Samuel was a faithful Prophet of the Lord, כִּ֚י נֶאֱמָ֣ן שְׁמוּאֵ֔ל לְנָבִ֖יא לַיהוָֽה׃] That Samuel was faithful unto a prophet for the Lord (Montanus, Septuagint), that is, that he might be a Prophet (Drusius). That Samuel was a faith prophet of Jehovah (Pagnine, similarly Tigurinus), or, in the prophetic words of the Lord (Jonathan in Drusius). To these, the ל/to/for in לְנָבִיא, to a prophet, is superfluous; as in the third, לְאַבְשָׁלוֹם/Absalom. To this pertains what follows in the Roman edition, ἐπιστεύθη Σαμουὴλ εἰς προφήτην γίνεσθαι τῷ Κυρίῳ, Samuel was believed to have been made into a prophet to the Lord. For, it is an interpretation of one of the ancient interpreters; perhaps of Symmachus (Drusius). That Samuel was faithful as a Prophet of the Lord (Munster). That Samuel was commissioned as a Prophet of God (Syriac). That Samuel had been constituted a Prophet by the Lord (Arabic). That he was faithful, etc., that is, that he was apt and suited, who at length would become a Prophet (Vatablus). That Samuel was faithful (or was thought faithful, or was found faithful [certain interpreters in the Dutch]), that he might be a Prophet (English in the Margin). That Samuel was established as a Prophet of Jehovah (Junius and Tremellius, Dutch, Drusius), or by the Lord (Drusius). The Greeks: πιστὸν, which signifies faithful, and also firm and stable. For, he was not a Prophet for a time only. Thus a house נֶאֱמָן, is firm (Drusius). That Samuel was certainly a prophet of Jove (Castalio). But in what manner did Israel come to know this? Responses: 1. From a prediction made, either by Samuel, or by Eli; who was indeed foretelling all those things that he had heard from God, with God Himself urging him, to all; that they should abstain from sins, and prepare themselves for future punishments, by the expiation of their crimes. 2. From the fulfillment of the things predicted; Whence, by anticipation, as Abulensis things, they are said in this place to have known, what afterwards, when it at length happened, they knew (Mendoza). God restored Prophecy, which had ceased with the impiety of those times; first to foretell ruin; then to direct the restoration of things: Thus God willed to supply the want of the Urim and Thummim, which were soon to be lost with the Ark (Lightfoot).
Knew, both by Eli’s testimony, and particular relation of the foregoing history, to the people that came from all parts; and by succeeding revelations made to him, whereof mention is made in the next verse, which though placed after, might be done before.
 Hebrew: וַיֵּ֙דַע֙ כָּל־יִשְׂרָאֵ֔ל מִדָּ֖ן וְעַד־בְּאֵ֣ר שָׁ֑בַע כִּ֚י נֶאֱמָ֣ן שְׁמוּאֵ֔ל לְנָבִ֖יא לַיהוָֽה׃  Hebrew: נֶאֱמָן.  1 Chronicles 3:2: “The third, Absalom (הַשְּׁלִשִׁי֙ לְאַבְשָׁל֣וֹם) the son of Maachah the daughter of Talmai king of Geshur: the fourth, Adonijah (הָרְבִיעִ֖י אֲדֹנִיָּ֥ה) the son of Haggith…”  It appears that this refers to the Roman, or Sixtine (having been authorized by Sixtus V), edition of the Septuagint, published in 1585 to assist in the preparation of the new Vulgate edition ordered by the Council of Trent.  For example, 2 Samuel 7:16: “And thine house and thy kingdom shall be established (וְנֶאְמַ֙ן בֵּיתְךָ֧ וּמַֽמְלַכְתְּךָ֛) for ever before thee: thy throne shall be established for ever.”