The Holy Trinity. The Orthodox Church teaches that God is triune - three divine persons (hypostases) who share one essence (ousia) - which leads us to describe God as coeternal and one/same essence, the term for which is homoousios. In other words, at no point in time (for God is beyond time) did any person of the Trinity not exist - God is before time, beyond time, and yet works within time, moving through His Church and speaking through His prophets. St. Gregory the Theologian tells us that we should approach God apophatically (to describe by negation) - to encounter God personally through participation in the Holy Mysteries of His Church, ultimately accepting the inadequacy of our limited human intellect to fully comprehend Him.
Two podcast episodes you may find helpful about this topic: