Joshua 23:10
One man of you shall chase a thousand: for the LORD your God, he it is that fighteth for you, as he hath promised you. One man of you shall chase a thousand: for the LORD your God, he it is that fighteth for you, as he hath promised you.
The verse belongs to Joshua's closing address to Israel's leaders after the wars of conquest when the Lord had given the people rest from their enemies, reviewing God's providence through their travels. That review was meant to awaken gratitude for God's providence and humility over failures to follow His will. Israel's strength and victories came not from their own strength or numbers but because the Lord Himself fought for them as their strength, support, and defense, in fulfillment of His promised help, so that one man could chase a thousand.
The promise operated when Israel remained faithful and obedient to God's word; prevailing power comes not from numbers but from walking with Him by holding firmly to His word, obeying Him, and claiming His promises. If Israel turned from God to idolatry and close ties with idolaters, He would withdraw His protection. God is faithful both to fulfill His promises and to carry out His warnings, so departure would bring desolation as enemies afflicted them and drove them from the land. His people must therefore avoid relationships that lead them away.
The same principle applies to devoted Christians now, so that a small number walking with God can prevail against far greater opposition because He fights for them. Each person should heed God's revealed will and avoid even the borders of sin.