Gregory the Wonderworker

Our father among the saints Gregory the Wonderworker , also known as Gregory Thaumaturgus or Gregory of Neocaesarea , (ca. 213 – ca. 270 AD) was a Christian bishop of the 3rd century. He is commemorated by the Church on November 17 .
The Theotokos and Apostle John appeared to St. Gregory in a dream, and taught him about the Holy Trinity . St. Gregory wrote down what was revealed to him.

Source: OrthodoxWiki