This verse alludes to another natural cause of the Muslim's failure in the battle of OHUD in order to reveal the weak points and amend them:
``So that Allah through that incident might purge those who are true in faith, and meanwhile blot out the infidels.'' The purge or separating and carrying off whatever is impure, was in two aspects: 1) It cleared out the hypocrite from the ranks of the Muslim warriors, because in the battle of Ohud there was an open treachery on the part of 300 hypocrites led by ABDULLAH-IBN-ABISELUL who deserted Muslims troops.
2) That bloody failure and effective test strengthened the faith of the weak and wavering, by showing them the worth and value of order and discipline, in the battle-field and in all fields of living.