Bede

And the Philistines gathered together, etc. The Philistines in this place, inasmuch as they seek the aid of David and his men, represent the faithful from the Gentiles; but inasmuch as they scorn and reject him, they represent the unfaithful who oppose Christ. For they oppose both the unfaithful Jews and the heretically unfaithful, and the Catholics, Christians and pagans alike. Therefore it is fitting that the Philistines are said to have been encamped in Sunam, which is interpreted as 'crimson'. …

Commentary on Samuel ↗