An excerpt from his Against Heresies (Book 4)
In the beginning God created Adam, not because he needed man, but because he wanted to have someone on whom to bestow his blessings. Not only before Adam but also before all creation, the Word was glorifying the Father in whom he dwelt, and was himself being glorified by the Father. The Word himself said: Father, glorify me with that glory I had with you before the world was.
Nor did the Lord need our service. He commanded us to follow him, but his was the gift of salvation. To follow the Savior is to share in salvation; to follow the light is to enjoy the light. Those who are in the light do not illuminate the light but are themselves illuminated and enlightened by the light. They add nothing to the light; rather, they are beneficiaries, for they are enlightened by the light.
The same is true of service to God: it adds nothing to God, nor does God need the service of man. Rather, he gives life and immortality and eternal glory to those who follow and serve him. He confers a benefit on his servants in return for their service and on his followers in return for their loyalty, but he receives no benefit from them. He is rich, perfect and in need of nothing.
The reason why God requires service from man is this: because he is good and merciful he desires to confer benefits on those who persevere in his service. In proportion to God’s need of nothing is man’s need for communion with God.
This is the glory of man: to persevere and remain in the service of God. For this reason the Lord told his disciples: You did not choose me but I chose you. He meant that his disciples did not glorify him by following him, but in following the Son of God they were glorified by him. As he said: I wish that where I am they also may be, that they may see my glory.
and to the Son
and to the Holy Spirit:
as it was in the beginning,
is now, and will be forever. Amen