The Lord Jesus had promised that the Holy Spirit should come, not then for the first time to be the Indweller of the saints, but to kindle to a greater heat, and to fill with larger abundance the hearts that were dedicated to Him, increasing, not commencing His gifts, not fresh in operation because richer in bounty.
For the Majesty of the Holy Ghost is never separate from the Omnipotence of the Father and the Son, and whatever the Divine government accomplishes in the ordering of all things, proceeds from the Providence of the whole Trinity.
Therein exists unity of mercy and loving-kindness, unity of judgment and justice: nor is there any division in action where there is no divergence of will. What, therefore, the Father enlightens, the Son enlightens, and the Holy Ghost enlightens, and…both the Unity and the Trinity are at the same time revealed to us….
The fact, therefore, that, with the co-operation of the inseparable Godhead still perfect, certain things are performed by the Father, certain by the Son, and certain by the Holy Spirit, in particular belongs to the ordering of our Redemption and the method of our salvation.
For if man, made after the image and likeness of God, had retained the dignity of his own nature, and had not been deceived by the devil’s wiles into transgressing through lust the law laid down for him, the Creator of the world would not have become a Creature, the Eternal would not have entered the sphere of time, nor God the Son, Who is equal with God the Father, have assumed the form of a slave and the likeness of sinful flesh.
But because “by the devil’s malice death entered into the world,” and captive humanity could not otherwise be set free without His undertaking our cause, Who without loss of His majesty should both become true Man, and alone have no taint of sin, the mercy of the Trinity divided for Itself the work of our restoration in such a way that the Father should be reconciled, the Son should reconcile, and the Holy Ghost enkindle.
For it was necessary that those who are to be saved should also do something on their part, and by the turning of their hearts to the Redeemer should quit the dominion of the enemy, even as the Apostle says, “God sent the Spirit of His Son into our hearts, crying Abba, Father,” “And where the Spirit of the Lord is, there is liberty,” and “no one can call Jesus Lord except in the Holy Spirit.”
Leo the Great (c.400-461): Sermon 77, 1-2.