on the Church As Being the Kingdom of Heaven

Ambrose of Milan On the Christian Faith book 5 ch 7.150 (340-397 ad)
It is a good thing to be in the kingdom of Christ, so that Christ may be with us; as He Himself says: "Lo I am with you always, even unto the end of the world."
Ambrose of Milan On the Christian Faith book 5 ch 7 (340-397 ad)
Here we are the kingdom of Christ, and in Christ's kingdom. Hereafter we shall be in the kingdom of God, where the Trinity will reign together.