One early, clear indication of the doctrine of the personhood of the Holy Spirit appears in Tertullian's work, Against Praxeas, dated around AD 215,1 saying:
[W]hile the mystery of the dispensation is still guarded, which distributes the Unity into a Trinity, placing in their order the three Persons— the Father, the Son, and the Holy Ghost: three, however, not in condition, but in degree; not in substance, but in form; not in power, but in aspect; yet of one substance, and of one condition, and of one power, inasmuch as He is one God, from whom these degrees and forms and aspects are reckoned, under the name of the Father, and of the Son, and of the Holy Ghost.
Tertullian mentions the personhood of the Holy Spirit several times in this work, but one example is particularly clear, in reference to the use of the plural "we" in the Creation account:
[I]t was because He [that is, the Father] 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."
Origen, writing around the same time as Tertullian, also mentions of the will and personhood of the Holy Spirit numerous times. Several examples from Book 1 of De Principiis (~AD 215) will suffice:2
the Holy Spirit is an intellectual existence [Ch. 1]
From all which we learn that the person of the Holy Spirit was of such authority and dignity, that saving baptism was not complete except by the authority of the most excellent Trinity of them all [Ch. 3]
the place where we treated, to the best of our ability, of the persons of Father, Son, and Holy Spirit. [Ch. 7]
Looking at the Holy Spirit more broadly, it's clear that Christians have recognized the existence of the Holy Spirit since New Testament times, and there's evidence that they considered him equal with/on the same level with God the Father and God the Son, as in early renditions of the Apostles' Creed (AD 200)3 and other writings.
The concept of the Spirit's will and independence may be found in Justin Martyr's Dialogue with Trypho4 (~AD 160), and even the word "person," though his formulation is not as clear as Tertullian's:
And the Holy Spirit, either from the person of His Father, or from His own person, answers them, 'The Lord of hosts, He is this King of glory.'
References and notes:
- Dating the work is difficult, but this chronology suggests the ~AD 215 timeframe.
- Also available in Ante-Nicene Fathers, Volume 4
- Philip Schaff, Creeds of Christendom, Volume 2, p55.
- Dialogue with Trypho in Ante-Nicene Fathers, Volume 1, pg 213