What does it mean when we say that we participate in God?
Participation in God means sharing in the very life of God Himself. Thus our participation in God is "energetic" and not "essential": we participate in the life of God, or in the grace of God, in HIs light, power, love, wisdom, glory, etc; but not in His "essence", in HIs inner being. Only the Three Members of the Holy Trinity are united with One Another essentially, that is to say, in Their one essence.
Participation denotes our entering into union and communion with God Himself; that union which was lost when our forefathers, Adam and Eve, separated themselves from God through disobedience.
The Fall resulted first in a spiritual death, which was quickly followed by a physical death, because as St. Irenaeus of Lyons (d. c. 202) tells us, we were created for the vision of God - so the vision of God is our life, and the vision of God signifies our participation in God.
As Irenaeus explains, “Just as they who see the light are in the light, so it is that they who see God are in God, partaking in His brightness”.
According to St. Gregory and the Holy Fathers, the whole human being, body and soul, is created for this vision: to be filled with the life of God Himself. Our participation in God, therefore, is psychosomatic - including the body as well as the soul, as the Holy Transfiguration of the Saviour on Mount Tabor reveals most clearly.
For a summary of St. Irenaeus’ teaching on the Transfiguration, see my book The Transfiguration of Christ in Greek Patristic Literature: From Irenaeus of Lyons to Gregory Palamas (Dalton PA: 2022) pp. 38-43.