Hebrews 2:4 (Morphosyntax)

Greek (WH): συνεπιμαρτυροῦντος τοῦ θεοῦ σημείοις τε καὶ τέρασιν καὶ ποικίλαις δυνάμεσιν καὶ πνεύματος ἁγίου μερισμοῖς κατὰ τὴν αὐτοῦ θέλησιν

CSB: "At the same time, God also testified by signs and wonders, various miracles, and distributions of gifts from the Holy Spirit according to his will."

James Moffatt NT: "while God corroborated their testimony with signs and wonders and a variety of miraculous powers, distributing the holy Spirit as it pleased him."

This verse begins with a word that morphologically is present active participle genitive singular masculine (συνεπιμαρτυροῦντος). William L. Lane (Hebrews 1-8, WB Commentary) states that συνεπιμαρτυρέω is used regularly in nonbiblical Greek, but the LXX does not employ the word and its only GNT occurrence is Heb. 2:4. Lane proffers the meaning, “to bear witness at the same time." Bill Mounce supplies these definitions: "to join in according testimony; to support by testimony, to confirm, sanction." One other point about the present participle is that Lane thinks it connotes ongoing action that continued to be shown in the early Christian community's daily life.

συνεπιμαρτυροῦντος τοῦ θεοῦ is a genitive absolute (A.T. Robertson, Word Pictures in the New Testament). ESV translates: "while God also bore witness"; NWT 2013: "while God joined in bearing witness." Dana Harris (Hebrews) relates that συνεπιμαρτυρέω is legal nomenclature and the participle may logically correlate with ἐβεβαιώθη in Hebrews 2:3 even if the correlation is not strictly grammatical: moreover, the participle in the genitive absolute construction logically depends on ἐβεβαιώθη.

The language, σημείοις τε καὶ τέρασιν καὶ ποικίλαις δυνάμεσιν emphasizes divine wonders, the exhibited power of God, and "the rich variety of divine activity." See Paul Ellingworth, The Epistle to the Hebrews, page 142. Additionally, Harris observes that one finds a string of instrumental datives here with ποικίλαις modifying δυνάμεσιν.

The writer of Hebrews has a propensity for the correlative τε καὶ (see Hebrews 4:12; 5:1,7,14; 8:3); καὶ πνεύματος ἁγίου μερισμοῖς κατὰ τὴν αὐτοῦ θέλησιν-The Greek μερισμός occurs twice in the GNT (Hebrews 2:4; 4:12); the word is best understood in terms of distribution for Heb. 2:4, contra Henry Alford.

Both Paul Ellingworth and Harold Attridge consider πνεύματος ἁγίου to be an objective genitive, and Attridge adds (Hebrews, 67-68) that the "spirit" in this verse:

refers not to a divine hypostasis, but to an eschatological gift of God's power and life.68 Hebrews will describe the "holy spirit" as speaking through the scriptures.69 That personification involves a traditional paraphrase for referring to the divine origin of scripture.

The Greek text reads, καὶ πνεύματος ἁγίου μερισμοῖς ("and distributions of holy spirit"), but commentators normally interpret the "distributions" (μερισμοῖς) to be gifts imparted through/by the holy spirit--it is not the holy spirit itself that is being distributed. Luke T. Johnson explains (Hebrews, page 89):

The expression “distributions of the Holy Spirit according to his will,” in turn, most clearly echoes Paul’s language in 1 Cor 12:11. Speaking of the “variety of gifts” (diaireseis charismaton), Paul says, “all these are activated by the one and same Spirit who allots to each one individually just as the Spirit chooses.”

On the other hand, see James Moffatt, The Epistle to the Hebrews, page 20. Note his translation supra.

Amplified Bible:  "[and besides this evidence] God also testifying with them [confirming the message of salvation], both by signs and wonders and by various miracles [carried out by Jesus and the apostles] and by [granting to believers the] gifts of the Holy Spirit according to His own will."

κατὰ τὴν αὐτοῦ θέλησιν is likely a reference to God per the context (i.e., God's will); θέλησιν is accusative singular feminine and the object of κατὰ. See Harris, Hebrews. The suffix -σιν that occurs three times in Heb. 2:4 is probably for rhetorical effect, according to Harris.

While I do not agree with Daniel Wallace's Trinitarian theology, his article here is useful in some ways: https://bible.org/article/hebrews-23-4-and-sign-gifts