Faint not, neither grieve, for you shall prevail if you are believers.
[This] means when you support or launch an attack by means of [the help of] God, you should not be afraid of anything other than God. Nor should you faint or become weak, for surely help is from God and the One who prevails is God. Nothing is equal to God. Not even an atom or a particle [originates] from them.
His words if you are believers, that is, it is not right that the believer should be overshadowed by fear of anything other than God.