The first Church to be called “Christian”:

St. Ignatius of Antioch ca. 50-117

Wherever the bishop shall appear, there let the multitude [of the people] also be; even as, wherever Jesus Christ is, there is the Catholic Church. (Smyrnaeans 8)

The first Church to use antiphonal hymns:

We must now however make some allusion to the origin of this custom in the church of responsive singing. Ignatius third bishop of Antioch in Syria from the Apostle Peter, who also had held intercourse with the Apostles themselves, saw a vision of angels hymning in alternate chants the Holy Trinity. Accordingly he introduced the mode of singing he had observed in the vision into the Antiochian church; whence it was transmitted by Tradition to all the other churches. Such is the account [we have received] in relation to these responsive hymns. (Socrates Scholasticus, Church History Bk. 6.8)

That excellent pair Flavianus and Diodorus, though not yet admitted to the priesthood and still ranked with the laity, worked night and day to stimulate men’s zeal for truth. They were the first to divide choirs into two parts, and to teach them to sing the psalms of David antiphonally. Introduced first at Antioch, the practice spread in all directions, and penetrated to the ends of the earth. Its originators now collected the lovers of the Divine word and work into the Churches of the Martyrs, and with them spent the night in singing psalms to God. (Blessed Theodoret, Ecclesiastical History Bk.2.19)

The first recorded use of the Greek word “Trinity” in Christian theology:

Theophilus of Antioch died ca. 185

For the sun is a type of God, and the moon of man. And as the sun far surpasses the moon in power and glory, so far does God surpass man. And as the sun remains ever full, never becoming less, so does God always abide perfect, being full of all power, and understanding, and wisdom, and immortality, and all good. But the moon wanes monthly, and in a manner dies, being a type of man; then it is born again, and is crescent, for a pattern of the future resurrection. In like manner also the three days which were before the luminaries, are types of the Trinity, of God, and His Word, and His wisdom. (To Autolycus Bk. 2.15)