The Lord is not with Israel - "The Word of the Lord is not the helper of the Israelites, nor of the kingdom of the tribe of Ephraim." - Targum.

But there came a man of God to him, saying, O king, let not the army of Israel go with thee; for the LORD [is] not (e) with Israel, [to wit, with] all the children of Ephraim.
(e) Therefore to think to have help from them, whom the Lord does not favour, is to cast off the help of the Lord.

But there came a man of God to him,.... Who the Jews say (f) was the brother of the king, Amos, the father of Isaiah, but without foundation:
saying, O king, let not the army of Israel go with thee; to war against Edom; meaning the 100,000 men he had hired, and suggesting that to have them with him would not be to his advantage, but his detriment, for the following reason:
for the Lord is not with Israel, to wit:
with all the children of Ephraim; they being idolaters, worshippers of the calves; and as they had forsaken the Lord, he had forsaken them; therefore no help could be expected from them; the Targum is,"the Word of the Lord is not their help.''
(f) Seder Olam Rabba, c. 20. Shalshalet Hakabala, fol. 11. 2.

there came a man of God--sent to dissuade Amaziah from the course he was following, on the ground that "the Lord is not with Israel." This statement was perfectly intelligible to the king. But the historian, writing long after, thought it might require explanation, and therefore added the comment, "with all the children of Ephraim." Idolatry had long been the prevailing religion in that kingdom, and Ephraim its headquarters. As to the other part of the prophet's advice (2-Chronicles 25:8), considerable obscurity hangs over it, as the text stands; and hence some able critics have suggested the insertion of "not" in the middle clause, so that the verse will be thus: "But if thou wilt go [alone], do, be strong for the battle; God shall not make thee fall before the enemy."

Let not, &c. - It is comfortable to employ those, who we have reason to hope, have an interest in heaven, but dangerous associating with those from whom the Lord is departed.