I read this Psalm as the utterance of the Jewish Remnant exercising very lively faith in their Messiah in the day when He will take their trouble upon Himself, and come forth to assert His kingdom against His and their enemy. They accordingly commend Him to the care of Jehovah, and anticipate His victory, and that they themselves shall therefore, like their fathers (Ex. 17:15), have a banner in Jehovah, though in conflict with the true Amalek.
The people in this spirit commended Joshua to God’s care as he was going out to his battles (Josh. 1:17-18). And according to the divine ordinance, when Israel went out to battle, they were to encourage themselves in God, and not be afraid of the multitudes of the enemy, or of their chariots and their horses. (See Deut. 20:1.) Jesus, as one fully obedient to this ordinance, here goes forth to the warfare in this spirit.
In the full power of Psalm 20:3 we see our Lord leaving His priestly services in heaven, now that He is about to take this other service, this duty of “the God of battles,” the Redeemer of the inheritance, upon Him. And this present action, His going forth in due season against His enemies, had been pledged to Him as soon as He took His seat in heaven. (See Psa. 110:1.) And He had been expecting it (Heb. 10:13).