This is the first antiphon for the first psalm (30) for Wednesday Night Prayer in the Liturgy of the hours. It is difficult. What do we do with “in Deum protectorem”? Esto is an imperative (fancy form) of the verb esse, meaning “be”. We are speaking to God and telling Him to be something.

Then, mihi simply means for me. Be for me…something. What?

in Deum protectorem – I think we have to stretch the word “in”. When the preposition in is used with the accusative, as it is here, it implies physical or moral motion and is translated into, unto, toward, about. Whitaker gives us what is perhaps the key: “after (manner)”. Be .. for me.. in Deum protectorem -> after the manner of a protector God. What is meant here is simply this: be for me a Protector-God, a House of Refuge. At least that’s how I read it.