The Mekhilta reads the phrase "You have guided them in Your strength" as a prophecy pointing forward in time. God guided Israel through the sea not because of anything they had already done, but in the merit of the Torah they were destined to receive at Sinai. The redemption from Egypt was, in a sense, an advance payment on a future commitment.
The key to this interpretation lies in the word "strength." The rabbis identified "strength" as a code word for Torah throughout scripture. The Psalmist writes (Psalms 29:4): "The Lord will give strength to His people; the Lord will bless His people with peace." And again (Psalms 99:4): "The strength of the King who loves justice." In both cases, "strength" refers not to military might but to the Torah itself — the divine teaching that would become Israel's true source of power.
This midrash (rabbinic interpretive commentary) presents a remarkable theological idea. God does not merely respond to past merit; He acts on future merit as well. The Israelites standing at the shore of the Red Sea had not yet heard a single commandment from Sinai. They had not yet studied a single law. Yet their future acceptance of the Torah was real enough, certain enough, that God treated it as grounds for deliverance right then and there.
The guidance through the wilderness was itself a form of Torah — strength given before strength was received.