[3:139] Faint not, neither grieve, for you shall prevail if you are believers.
It means when you support or launch an attack for God, you should not be afraid of anything other than God. Nor should you grieve or become weak, for surely victory is from God and the One who prevails is God. Nothing is equal to God, not so much as an atom or a sīna from them.
His words if you are believers: i.e., fear of anything other than God should not cast its shadow over the believer.