24 And it is rightly named (Ecclesia) because it calls forth and assembles together all men; according as the Lord says in Leviticus, And make an assembly for all the congregation at the door of the tabernacle of witness. And it is to be noted, that the word assemble, is used for the first time in the Scriptures here, at the time when the Lord puts Aaron into the High-priesthood. And in Deuteronomy also the Lord says to Moses, Assemble the people unto Me, and let them hear My words, that they may learn to fear Me. And he again mentions the name of the Church, when he says concerning the Tables, And on them were written all the words which the Lord spoke with you in the mount out of the midst of the fire in the day of the Assembly; as if he had said more plainly, in the day in which you were called and gathered together by God. The Psalmist also says, I will give thanks unto You, O Lord, in the great Congregation; I will praise You among much people.
Source: Catechetical Lectures (New Advent)