In Summa theologiae Ia, q. 13, a. 11, Saint Thomas identifies the most proper name of God. Thomas invokes God’s revelation to Moses as: “I am who I am”. According to Thomas, He-Who-Is (qui est) is the most proper name for God for three reasons. The first is that the phrase does not signify form, but existence itself (ipsum esse). The esse (existence) of God is the essentia (essence) of God. Since this identity holds only for God (and not for any created thing), then “qui est” is the most fitting name for God. Whereas most things are denominated by their form, God is not. God is named simply as esse ipsum.
This first argument relates to the first objection which objects that “qui est” cannot be the most proper name for God because it is not an incommunicable name. For example, a fish might also be it-who-is or “qui est”. However, in the case of a fish, “qui est” does not denote something that exists in itself, as it does with God. The divine essence is in fact incommunicable and so this name that denotes the essence is the most proper name for God.
The second argument is that “qui est” is the most universal name for God, because the less determinate a name is, the more universal the name is. Other names determine a mode of being, but “qui est” does not determine a mode of being. Thomas rather beautifully describes the name as denominating the “infinite ocean of substance” (pelagus substantiae infinitum). The third argument is that “qui est” denotes only a present existence without past or future. Where esse and essentia are the same, there can be neither past nor future.
Thomas’ argument is sound, but I think that he should have addressed another hypothetical objection: We are baptized into the name of the Father, and of the Son, and of the Holy Spirit. Our beatitude is found in the Trinitarian framework of redemption, and the New Testament doesn’t seem to invoke God readily as “qui est”.