"Wherefore also the Holy Spirit consists in the same unity of substance, and in the same equality. For whether He is the unity of both, or the holiness, or the love, or therefore the unity because the love, and therefore the love because the holiness, it is manifest that He is not one of the two, through whom the two are joined, through whom the Begotten is loved by the Begetter, and loves Him that begot Him, and through whom, not by participation, but by their own essence, neither by the gift of any superior, but by their own, they are 'keeping the unity of the Spirit in the bond of peace;' which we are commanded to imitate by grace, both towards God and towards ourselves" (De Trinitate VI.5.7).
Source: Translated by Arthur West Haddan. From Nicene and Post-Nicene Fathers, First Series, Vol. 3. Edited by Philip Schaff. (Buffalo, NY: Christian Literature Publishing Co., 1887.) Revised and edited for New Advent by Kevin Knight.