A Problem of Literal Interpretation of Texts in Old Testament

In Numbers 15:41, the Lord says, "I am the Lord your God, who brought you out of Egypt to be your God." If a literal interpretation of the text is made, it would be like God had never been the God of the Israelites before the Exodus because his being their God is stated in a future tense: "to be your God."

He said this right after giving Moses a command "to make tassels on the corners of their garments, with a blue cord on each tassel" (v. 38). He gave this command for the Israelites to "look at them" and not to commit immoral sins (v. 39), so that they "will remember to obey all [his] commands and will be consecrated to [their] God" (v. 40). No other context can be found in the text.

The danger of the literal interpretation might lead to an understanding that God was not truly the God for the Israelites while they were presumably, as the context suggests, leading immoral lives under the bondage of the slavery. This cannot be true.

Of course, we can find many Scriptural evidences for God's temporally-unbound lordship over Israel. He made an "everlasting" covenant with Abraham (Gen. 17:7), and prescribed circumcision as a sign that every male should undergo "for generations to come" (v. 12). But does this passage override the literal meaning of the text in Numbers 15:41? If so, what was the purpose of the Lord speaking his lordship in the future tense? The best answer might be that God can be truly God when his people obey his commands and consecrate themself to God. This does not make sense because his lordship existed even when they were under the slavery and presumably under sinful states in Egypt.

How can we interpret a biblical passage right?
We must have some fundamental and universally-accepted systematic theological doctrines ready before reading a text to interpret. And from the get-go, spiritualization is to be made. Presupposition is inevitable for hermeneutics and it does not necessarily impede the interpretation. Rather it is necessary. A good analogy may be the image of groping an elephant by a blind man. He would never know the true shape and figure of the elephant without having an image of elephant. Without overarching principles of the Christianity, literal interpretation may give a false meaning of the text. With true theological principles, biblical interpretation can bring out consistent and true meanings of texts.