Trinity
In the fourth-century, Marcellus of Ancyra declared that the idea of the Godhead existing as three hypostases came from Plato, through the teachings of Valentinus. Valentinus is quoted as teaching that God is three, three prosopa (persons) called the Father, the Son and the Holy Spirit:
These men also taught three hypostases, just as Valentinus the heresiarch first invented in the book entitled by him 'On the Three Natures'. It was believed he was the first to invent three hypostases and three persons of the Father, Son and Holy Spirit, but he was discovered to have taken this from Hermes and Plato.
Valentinus (also spelled Valentinius) (c.100 - c.160) was known as a early Christian Gnostic Theologian.
It should be noted that Nag Hammadi library Sethian text such as Trimorphic Protennoia identify Gnosticism as also professing Father, Son and feminine wisdom Sophia or as Professor John D Turner denotes, God the Father, Sophia the Mother, and Logos the Son.