bai,
the early church Fathers or early Christians already held the Trinitarian view on God even before the council of Nicea..
Ignatius of Antioch
"To the Church at Ephesus in Asia . . . chosen through true suffering by the will of the Father in Jesus Christ our God" (Letter to the Ephesians 1 [A.D. 110]).
"For our God, Jesus Christ, was conceived by Mary in accord with God’s plan: of the seed of David, it is true, but also of the Holy Spirit" (ibid., 18:2).
Justin Martyr
"We will prove that we worship him reasonably; for we have learned that he is the Son of the true God himself, that he holds a second place, and the Spirit of prophecy a third. For this they accuse us of madness, saying that we attribute to a crucified man a place second to the unchangeable and eternal God, the Creator of all things; but they are ignorant of the mystery which lies therein" (First Apology 13:5–6 [A.D. 151]).
Theophilus of Antioch
"It is the attribute of God, of the most high and almighty and of the living God, not only to be everywhere, but also to see and hear all; for he can in no way be contained in a place. . . . The three days before the luminaries were created are types of the Trinity: God, his Word, and his Wisdom" (To Autolycus 2:15 [A.D. 181]).
Irenaeus
"For the Church, although dispersed throughout the whole world even to the ends of the earth, has received from the apostles and from their disciples the faith in one God, the Father Almighty . . . and in one Jesus Christ, the Son of God, who became flesh for our salvation; and in the Holy Spirit" (Against Heresies 1:10:1 [A.D. 189]).
source:
The Trinity | Catholic Answers
feel free to verify with neutral sites or with history concerning the writings of the Church Fathers for you to cross reference..