Is Yahweh the Christian God?

There are professing Christians who essentially reject OT theism. They treat OT Judaism as a different religion. What are the differences between the OT and the NT?

i) In a broad sense, the OT represents promise while the NT represents fulfillment. Mind you, even that contrast presumes underlying continuity. The NT can only fulfill the OT if the OT is true. 

ii) In addition, this contrast is somewhat overstated. Both the OT and the NT contain eschatological promises whose fulfillment remains future. Some OT prophecies are fulfilled in the NT, but others remain outstanding. 

iii) The NT contains far less military and political history than the OT. The OT contains extensive historical narratives of palace intrigue, civil war, siege-warfare, and war with Israel's neighbors. It also narrates conditions of cyclical national apostasy. So there's a lot of violence and unsavory material in these pages. 

The four Gospels and Acts are the main historical books in the NT. The Gospels are tightly focussed on the public ministry of Christ while Acts is tightly focused on church-planting. If the NT devoted as much attention to narrating life in the Roman Empire that the OT does to life in the ANE, the NT would be just as violent and unsavory. 

The difference has to do with selection-criteria. The OT has more occasion to narrate brutality and depravity. It narrates the history of a nation. 

iv) Because Israel was a nation-state, the OT includes a law code. The law code includes a penal code as well as laws of war. 

This doesn't make for pleasant reading. However, it's not as if the new covenant obviates the need for a penal code or national defense. Most Christians aren't pacifists or anarchists. So Christian social life will require a counterpart to the OT law code. A penal code and military. 

I daresay that many Christian pacifists are only pacifists on paper. If an armed assailant broke into their home, threatened to rape their wife and slit the throats of their children, their theoretical pacifism would fly out the window. This is just a radical chic pose. 

v) The OT also contains purity codes, the violation of which can result in capital punishment or direct divine execution. The NT lacks the same purity codes. However, there's a principle which carries over. If anything, it's heightened: 

29 How much worse punishment, do you think, will be deserved by the one who has trampled underfoot the Son of God, and has profaned the blood of the covenant by which he was sanctified, and has outraged the Spirit of grace? 30 For we know him who said, “Vengeance is mine; I will repay.” And again, “The Lord will judge his people.” 31 It is a fearful thing to fall into the hands of the living God (Heb 10:29-31). 

vi) The fatal judgment exacted on Ananias, Sapphira (Acts 5), and Herod Agrippa (Acts 12), is very reminiscent of divine violence in the OT. Although it's not on the same scale as some OT judgments, that's a difference in degree rather than kind. 

vii) Likewise, the sack of Jerusalem, which Jesus threatens against apostate Jerusalem, is an example of large-scale divine violence, comparable to many OT judgments. 

viii) The NT contains a certain amount of martial imagery in describing the final judgment. I don't think Jesus literally swoops down from heaven on a war horse and puts his enemies to the sword (Rev 19). However, Revelation  does involve the forcible subjugation of God's enemies. And we can't rule out the possibility that this involves physical violence.