John Owen's masterpiece On Communion with God the Father, Son and Holy Ghost was written at a time when Socinianism (a form of Unitarianism) was infiltrating England. Their belief (as expressed in the Racovian Catechism) was that Jesus was essential for salvation. He was manifestly predicted and prophesied in the OT. The Hebrew Scriptures were indeed a word about Christ. But, for the Socinians, Christ existed before his birth only inasmuch as God always had a plan (or 'word') which Christ fulfilled in the NT ('was made flesh'). Christ's pre-existence then was not as a distinct, concrete Person in the Godhead, but as a saving/revealing disposition belonging to the one God of Israel. Thus Jesus was not the eternal word/wisdom/revelation of God but only the ultimate word/wisdom/revelation of God.
John Owen considered this to be a foul assault on the divine Person of Christ. This was a re-incarnation of Arianism - the great heresy of heresies. Perhaps his major response was Christologia in which one of his key arguments is that the OT also reveals Christ as a 'distinct Person within the deity.' (a repeated phrase). Perhaps we'll look at that book another time. But for now let's look at Communion with God penned 20 years earlier.
His main premise is that there is a distinct and distinguishable communication of grace coming from each Person of the Trinity. The saints should therefore have distinct communion with each Person of the Trinity individually. The rest of the book unfolds the ways in which we hold communion with the Father, the Son and the Spirit.
What's interesting for our current purposes is that Owen argues for this distinct experience of each Person from both testaments. According to Owen the OT also reveals the distinct Persons in their distinct roles.
I will list his OT Scriptures regarding the distinct Person of the Son. I am not including his verses on the Song of Songs or verses teaching more general truths about God's character. But these, according to Owen, are specific verses about the Son :
I hope you see the importance of these verses. Owen uses these as proof texts that the Son is distinct and known as distinct from the Father and Spirit. Owen's argument doesn't work if they're just verses about 'God' in general and 'hey, Jesus happens to be God too!' It's about proving from all of Scripture that the Son is revealed in His deity and distinction.
I maintain that it's this kind of biblical theology that will protect us from unitarian pressures in our own day.