Ireneaus was the first to use the term Trinity
Irenaeus wrote about the triune nature of God.
That is the essential element of the doctrine of the Trinity - the distinct role of each member of the Godhead and its relationship to all of humanity.
The Trinity is one God in three persons:
God the Father - creator of the universe.
The Son of God - the Word of God, our Lord Jesus Christ.
The Holy Spirit - through whom the prophets prophesied.