In his recent debate with David Wood, Muhammad Hijab challenged David to produce evidence that the church fathers before Nicaea believed in the divinity of the Holy Spirit. David didn’t really have a response to this challenge, and I suspect that other Muslims will attempt to use this argument against Christians in the future. To pre-empt this, here are some quotes from Justin Martyr’s First Apology (mid 2nd century) where he treats the Holy Spirit as a divine person:

Hence are we called atheists. And we confess that we are atheists, so far as gods of this sort are concerned, but not with respect to the most true God, the Father of righteousness and temperance and the other virtues, who is free from all impurity. But both Him, and the Son (who came forth from Him and taught us these things, and the host of the other good angels who follow and are made like to Him), and the prophetic Spirit, we worship and adore, knowing them in reason and truth, and declaring without grudging to every one who wishes to learn, as we have been taught. (Justin Martyr, First Apology, ch. 6)

Further on, he says:

Our teacher of these things is Jesus Christ, who also was born for this purpose, and was crucified under Pontius Pilate, procurator of Judæa, in the times of Tiberius Cæsar; and that we reasonably worship Him, having learned that He is the Son of the true God Himself, and holding Him in the second place, and the prophetic Spirit in the third, we will prove. For they proclaim our madness to consist in this, that we give to a crucified man a place second to the unchangeable and eternal God, the Creator of all; for they do not discern the mystery that is herein, to which, as we make it plain to you, we pray you to give heed. (Justin Martyr, First Apology, ch. 13)

J.N.D. Kelly writes of Justin Martyr’s Pneumatology as follows:

On several occasions Justin coordinates the three Persons, sometimes quoting formulae derived from baptism and the eucharist, and at other times echoing official catechetical teaching. Thus he counters the charge of atheism brought against Christians by pointing to the veneration they pay to the Father, the Son and ‘the prophetic Spirit’. Indeed, references to ‘the holy Spirit’ or ‘the prophetic Spirit’ abound in his writings; and although he was often hazy about the relation of His functions to those of the Logos, the attempts he made3 to extract testimony to His existence as a third divine being from Plato’s writings prove that he regarded the two as really distinct. (J. N. D. Kelly, Early Christian Doctrines, Fifth, Revised. (London; New Delhi; New York; Sydney: Bloomsbury, 1977), 102.)