Old Testament readings use the Septuagint , the Scripture the apostles quoted. Masoretic numbering shown for reference.Learn why
LXX

2 Chronicles 20:15

and he said, Hear ye, all Juda, and the dwellers in Jerusalem, and king Josaphat: Thus saith the Lord to you, even you, Fear not, neither be alarmed, before all this great multitude; for the battle is not yours, but God's.

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