It shall not be you that shall fight, but only stand with confidence, and you shall see the help of the Lord over you, O Juda, and Jerusalem: fear ye not, nor be you dismayed: to morrow you shall go out against them, and the Lord will be with you.
2 Chronicles 20:15 And he said: Attend ye, all Juda, and you that dwell in Jerusalem, and thou king Josaphat: Thus saith the Lord to you: Fear ye not, and be not dismayed at this multitude: for the battle is not yours, but God's.
2 Chronicles 20:16 To morrow you shall go down against them: for they will come up by the ascent named Sis, and you shall find them at the head of the torrent, which is over against the wilderness of Jeruel.
2 Chronicles 20:17 It shall not be you that shall fight, but only stand with confidence, and you shall see the help of the Lord over you, O Juda, and Jerusalem: fear ye not, nor be you dismayed: to morrow you shall go out against them, and the Lord will be with you.
2 Chronicles 20:18 Then Josaphat, and Juda, and all the inhabitants of Jerusalem fell flat on the ground before the Lord, and adored him.
2 Chronicles 20:19 And the Levites of the sons of Caath, and of the sons of Core praised the Lord the God of Israel with a loud voice, on high.
The verse centers on "shall", "fight", "only", "stand", "confidence", and "help". It is saying that the verse draws attention to "shall" and "fight", so its meaning should be read from those terms before moving to application.
The nearby context moves from verse 16's "To morrow you shall go down against..." into verse 18's "Then Josaphat and Juda and all the...", so "shall" and "fight" belong inside that flow. In 2 Chronicles context, the local focus is covenant, worship, and faithfulness.
A plain takeaway is to answer the verse's own emphasis on "shall" and "fight" with trust shaped by these words, not by a vague optimism outside the passage.