The crown prince

One of the perceived challenges for Christians is how to account for the seemingly abrupt transition from OT monotheism to NT Trinitarianism. How could NT writers go so easily from one to another? I think that problem is fairly artificial, because it overstates the difference. 

i) To begin with, the OT has the Spirit of God in addition to Yahweh. But surely Jews understood the Spirit of God to be divine. Not a creature. 

At the same time, spirits are personal agents in the world of the Bible, viz. angels, demons, ghosts. So, by the same token, it would be natural for Jews to understand the Spirit of God as a personal agent. 

In that respect, OT Judaism was already binitarian rather than unitarian. Jews may not have mentally sorted out exactly how Yahweh and the Spirit of God were interrelated, but it's not as if there's a chasm to leap across as we move from OT theism to NT theism. 

ii) Moreover, the basic reason the NT is more explicit about the Trinity is that it tells the story of the Father sending the Son into the world to redeem sinners. And that, in turn, is followed by the Father (or the Son) sending the Spirit into the world to take up where Jesus left off. Since that operation hadn't happened in OT times, there was no need to spell it out. The OT mentions different elements of that scenario, but doesn't put it all together.

The NT pulls back the curtain to reveal what was going on behind-the scenes. And that's necessary to explain the significance of Christ's mission. 

iii) The other element you have is the divine messiah. And it wasn't much of an adjustment for NT writers to believe in a divine messiah.

Christian apologists refer to certain prooftexts like Isa 9:6, and that's a valid appeal. However, too much emphasis on Isa 9:6 can be misleading, as if it's rare for the OT to anticipate a divine messiah.

But that overlooks a common OT motif. In the OT generally, there's the oft-repeated story of a coming prince. He is the crown prince. The heir apparent. The royal son of his regal father. 

There are many versions of this story in the OT. And all of them imply a divine messiah. In the story, the king stands for God. Hence, the prince stands for God's son. 

A son succeeds his father because he is most like his father. No one else is more like, or even as much like, a man than his son.

But in this story, the king is divine–which makes the son divine. There's a necessary and essential parity between the nature of the reigning monarch, and the nature of the heir who takes his place, or reigns alongside him.