Adnan asked where in the early church fathers did they mention the Deity of the Holy Spirit? Here are a couple of good quotes on that from Tertullian, who wrote this book, Against Praxeas, against what is called Modalism/Sabellianism / Monarchianism (that the Father became the Son and then the Son became the Spirit, that they are one person with different masks on at different times, and that the Father suffered, “Patri-passionism”), written around 208 AD.
These are famous passages where Tertullian uses the Latin words, Trinitas and Unitas and three persona (for three persons, which in Greek, the equivalent was hypostasis / ‘υποστασις ). * see correction below.
* Addendum and Correction, thanks to David Waltz of Articuli Fidei blog:
“The Greek equivalent for persona in Tertulllian’s day was πρόσωπον. [prosopon = face of a person, countenance, presence; hence, “person, personality”] It was not until after the Nicene Council of 325 that the Eastern/Greek Church Fathers consistently replaced πρόσωπον with ὑπόστασις (especially so, through the efforts of the three Cappadocians).” [my comments added in black]
I actually knew that before, but forgot; thanks again David for your accuracy and knowledge of church history and the early church fathers!
If the number of the Trinity also offends you, as if it were not connected in the simple Unity, I ask you how it is possible for a Being who is merely and absolutely One and Singular, to speak in plural phrase, saying, “Let us make man in our own image, and after our own likeness;” whereas He ought to have said, “Let me make man in my own image, and after my own likeness,” as being a unique and singular Being? In the following passage, however, “Behold the man is become as one of us,” He is either deceiving or amusing us in speaking plurally, if He is One only and singular. Or was it to the angels that He spoke, as the Jews interpret the passage, because these also acknowledge not the Son? Or was it because He was at once the Father, the Son, and the Spirit, that He spoke to Himself in plural terms, making Himself plural on that very account? Nay, it was because He had already His Son close at His side, as a second Person, His own Word, and a third Person also, the Spirit in the Word, that He purposely adopted the plural phrase, “Let us make;” and, “in our image;” and, “become as one of us.” For with whom did He make man? and to whom did He make him like? (The answer must be), the Son on the one hand, who was one day to put on human nature; and the Spirit on the other, who was to sanctify man. With these did He then speak, in the Unity of the Trinity, as with His ministers and witnesses. In the following text also He distinguishes among the Persons: “So God created man in His own image; in the image of God created He him.”
Tertullian, Against Praxeas, XII (Bold my emphasis)
Chapter XXV.—The Paraclete, or Holy Ghost. He is Distinct from the Father and the Son as to Their Personal Existence. One and Inseparable from Them as to Their Divine Nature. Other Quotations Out of St. John’s Gospel.
“What follows Philip’s question [ John 14:5-9, see Against Praxeas XXIV, in previous paragraph ] , and the Lord’s whole treatment of it, to the end of John’s Gospel, continues to furnish us with statements of the same kind, distinguishing the Father and the Son, with the properties of each. Then there is the Paraclete or Comforter, also, which He promises to pray for to the Father, and to send from heaven after He had ascended to the Father. He is called “another Comforter,” indeed; but in what way He is another we have already shown, “He shall receive of mine,” says Christ, just as Christ Himself received of the Father’s. Thus the connection of the Father in the Son, and of the Son in the Paraclete, produces three coherent Persons, who are yet distinct One from Another. These Three are one essence, not one Person, as it is said, “I and my Father are One,” in respect of unity of substance not singularity of number. Run through the whole Gospel, and you will find that He whom you believe to be the Father (described as acting for the Father, although you, for your part, forsooth, suppose that “the Father, being the husbandman,” must surely have been on earth) is once more recognized by the Son as in heaven, when, “lifting up His eyes thereto,” He commended His disciples to the safe-keeping of the Father. We have, moreover, in that other Gospel a clear revelation, i.e. of the Son’s distinction from the Father, “My God, why hast Thou forsaken me?” and again, (in the third Gospel,) “Father, into Thy hands I commend my spirit.” But even if (we had not these passages, we meet with satisfactory evidence) after His resurrection and glorious victory over death. Now that all the restraint of His humiliation is taken away, He might, if possible, have shown Himself as the Father to so faithful a woman (as Mary Magdalene) when she approached to touch Him, out of love, not from curiosity, nor with Thomas’ incredulity. But not so; Jesus saith unto her, “Touch me not, for I am not yet ascended to my Father; but go to my brethren” (and even in this He proves Himself to be the Son; for if He had been the Father, He would have called them His children, (instead of His brethren), “and say unto them, I ascend unto my Father and your Father, and to my God and your God.” Now, does this mean, I ascend as the Father to the Father, and as God to God? Or as the Son to the Father, and as the Word to God? Wherefore also does this Gospel, at its very termination, intimate that these things were ever written, if it be not, to use its own words, “that ye might believe that Jesus Christ is the Son of God?” Whenever, therefore, you take any of the statements of this Gospel, and apply them to demonstrate the identity of the Father and the Son, supposing that they serve your views therein, you are contending against the definite purpose of the Gospel. For these things certainly are not written that you may believe that Jesus Christ is the Father, but the Son.“
Tertullian, Against Praxeas, XXV (Bold my emphasis)