What is the earliest clear and unambiguous post-NT affirmation of the Holy Spirit's personhood in Christianity?

Tertullian (A.D. c. 155 – c. 220), was the first Latin church father who spoke of ''three persons'' (Latin: "tres Personae'') and he himself believed that the Holy Spirit has personal existence just as the Father and the Son are.

''Tertullian is recognized as one of the earliest theologians of the Latin Church. He responded to a number of theological issues that arose during his lifetime. He argued against Marcion and other Gnostics. Most importantly for this discussion he presented one of the first extensive defenses of the Trinity in his writings. François Decret praises Tertullian writing, “. . .Tertullian’s trinitarian doctrine was a decisive contribution to orthodox theology, as he was the first Latin writer to use the term trinitas to refer to the three persons of the godhead.”2 Tertullian’s contribution to the development of Trinitarian doctrine went much further than simply the use of the term trinitas. Tertullian’s major contribution to the doctrine of the Trinity is his use of the terms “substance” (substantia) and “person” (personae). ( Substance and Person in Tertullian and Augustine Andrew P. Hillaker, 2018).

''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. What follows Philip's question, 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; John 14:16 but in what way He is another we have already shown, He shall receive of mine, says Christ, John 16:14 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, John 10:30 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, John 15:1 must surely have been on earth) is once more recognised by the Son as in heaven, when, lifting up His eyes thereto, John 17:1 He commended His disciples to the safe-keeping of the Father. John 17:11 We have, moreover, in that other Gospel a clear revelation, i.e. of the Son's distinction from the Father, My God, why have You forsaken me? Matthew 27:46 and again, (in the third Gospel,) Father, into Your hands I commend my spirit. Luke 23:46'' (Against Praxeas, Chapter 25).

Among the Greek church fathers, it was Origen of Alexandria (c. 185–c. 253) who first identified the Holy Spirit as a person (Greek: hypostasis). Note that Origen said that there was never a time when wisdom was latent thought of the Father and had not yet come forth as speech. Origen did not believe in the distinction between the logos endiathetos and logos prophorikos which some of the 2nd century Greek fathers (e.g. Tertullian) believed in. For Origen, the Son of God is always a person. In the very context where the Holy Spirit is also called a person, together with the Father. This showed that Origen truly believed that the Holy Spirit is a hypostasis. He even termed the three as ''three hypostases''.

On the other hand, when we read that the Son is the wisdom and power of the Father (1 Corinthians 1.24) and that the world was created through him (Hebrews 1.2), we are to understand that he is that divine helpmate who declares at Proverbs 8.22 that the Lord created her in the beginning of his ways, and at Wisdom7.26 that she is the mirror of his unspotted majesty. The verb “created” in this text (which Origen prefers to the alternative reading “possessed”) does not imply that the Son has a temporal beginning, but that, having no other substrate than the Father’s will, he expresses that will more perfectly than the things that are “made” from matter. It is inconceivable that the Father could ever have lacked wisdom, and equally inconceivable to Origen that this wisdom could ever have taken a different form from the one that it now possesses as the second person or hypostasis of the Trinity (Princ. 1.2.2). He is the first theologian to state unequivocally that the “three hypostases” which constitute the Trinity are eternal not only in nature, but in their hypostatic character; there was never a time when wisdom was the latent thought of the Father and had not yet come forth as speech. (Origen, Doctrine of God, Stanford Encyclopedia, 2018).

These church fathers (circa 2nd century A.D. - 3d century A.D.) believed that persona or hypostasis refers to a ''person'' numerically distinct from another person. Persona and hypostasis is synonymous with ousia, ousia being also referring to a person. Ousia as person refers to the ''primary ousia'' (the substance that is individuated in a particular subject i.e. for example, one with divine nature is called a ''divine person'', which is the substance individuated in a particular subject). Persona or hypostasis is what what later church fathers would call 'rational substance''. In the Nicene Creed (4th century A.D.), ousia and hypostasis were also synonymous. However, it had the ''secondary ousia'' meaning (i.e. substance common to the persons, which the 4th century fathers called ''species''). For more info about the primary ousia and secondary ousia in the early church, i highly recommend to read: How do the Father and Son differ if they have the same particular essence?

''The study proposes an analysis of the concepts of ousia and hypostasis in the theology of the Council of Antioch which condemned Paul of Samosata in 268 CE. The authentic reports preserved from the assembly unveil the fact that the synodals who condemned Paul of Samosata employed the two terms interchangeably to denote the individual entity or person rather than the common essence or nature of the Father and Son. Additionally, they defended Christ's divinity before time and simultaneously assumed a certain subordinationism. The study additionally explores the Sitz im Leben of this theology, an accepted language embraced in the Eastern part of the Roman world in the third century. The article further traces the elements of this Antiochene theology in the fourth century in what was traditionally viewed as the " Arian " councils held in Antioch in 341 and 345 as well as in such authors as Eusebius of Caesarea and the Homoiousians. While Antioch 341 and 345 distanced themselves from Arianism, it is more coherent to interpret them, together with Eusebius and the Homoiousians, through this new hermeneutical lens, namely Antioch 268, rather than the traditional polarization between Nicaea and Arianism.'' (Antioch 268 and Its Legacy in the Fourth- Century Theological Debates, Dragos Andrei Giulea, 2018, Harvard Theological Review)