39 For whereas he has specially ascribed to God that all things are from Him, and he has assigned as a peculiar property to Christ, that all things are through Him, and it is now the glory of God that from Him and through Him and in Him are all things; and whereas the Spirit of God is the same as the Spirit of Christ, or whereas in the ministration of the Lord and in the working of God, one Spirit both works and divides, They cannot but be one Whose properties are those of one; since in the same Lord the Son, and in the same God the Father, one and the same Spirit distributing in the same Holy Spirit accomplishes all things.
How worthy is this saint of the knowledge of exalted and heavenly mysteries, adopted and chosen to share in the secret things of God, preserving a due silence over things which may not be uttered, true apostle of Christ! How by the announcement of his clear teaching has he restrained the imaginations of human wilfulness, confessing, as he does, one God the Father and one Lord Jesus Christ, so that meanwhile no one can either preach two Gods or one solitary God; although He Who is not one person cannot multiply into two Gods, nor on the other hand can They Who are not two Gods be understood to be one single person; while meantime the revelation of God as Father demonstrates the true nativity of Christ.
Source: On the Trinity (New Advent)