The argument goes like this. Yes, it is granted, Jesus prohibits violence in the Sermon on the Mount. But we also see God acting violently in the Old Testament. Consequently, it is argued, we must remember that "it's the same God." The God of Jesus is the same God in the Old Testament. Thus, we can't assume that God is wholly and always opposed to war.
Further, to make the "it's the same God" argument even stronger (and nerdier) you can go on to accuse pacifists as being Marcionites.
If you don't know about the Marcion heresy a brief primer. Marcion of Sinope was a Christian bishop who lived, give or take, between 85 CE and 160 CE. Marcion was one of the first big heretics. And his heresy was this. Marcion thought that the Jewish god of the Old Testament was a different god from the god revealed to us by Jesus. According to Marcion the teachings of Jesus were incompatible with the actions of the god portrayed in the Old Testament. More, the Old Testament portrays the Jewish god as the Creator god (Marcion called this Creator god the Demiurge) and as seems obvious, this Creator god really made a mess of things. Just look how screwed up this world is.
Christian orthodoxy eventually rejected the Marcion heresy. You see this clearly in the first line of the Nicene Creed:
We believe in one God, the Father Almighty, Maker of heaven and earth...We believe in one God, not two gods (one from the Old Testament and a second from New Testament). And we believe that the God who made heaven and earth wasn't the Demiurge but the God confessed by Jesus Christ.
In short, the God of the Old Testament is the same God in the New Testament. If you're an orthodox Christian you have to reconcile with the Old Testament, as hard as that may be.
And here's my point. It is hard to reconcile the Old Testament God with the New Testament God. Marcion wasn't crazy. He was working through a problem we are still struggling with. Personally, I'm very sympathetic to Marcion. I've scratched my head a plenty in reading the Old Testament. And who hasn't read the Sermon on the Mount and wondered about the violence in the Old Testament?
And this brings me to the crux of this post. The creeds didn't really fix this problem. The creeds just affected the shape of the solution. The creeds basically ruled out a metaphysical fix to the problem. You can't reconcile--within orthodoxy--the Old and New Testaments by positing two gods. But that doesn't mean there isn't some hard reconciling still to do. The disjoints between the Old and New Testament haven't gone away. So how to we overcome the contradictions and tensions? The creeds say you can't fix it metaphysically, so you'll have to do it hermeneutically.
That's my point. The creeds don't fix the problem as much as they hand over to us a hermeneutical challenge. We confess that there is one God, Maker of heaven and earth. But how we make sense of that confession, in light of the Old and New Testaments, is hard, hard interpretive work.
(And incidentally, this is one of the beauties of the creeds. The creeds never went in for parsimony and logical coherence. The creeds always opted to make things stranger and harder. This is a point that Rowan Williams likes to make: theology should make conversation about God harder rather than easier.)
Let's now go back to the issue of war and the use of the Old Testament of justify war.
We can see, now, the shape of the Marcion accusation toward pacifists. When pacifists pit Jesus against YHWH in the Old Testament they are of accused of Marcionism because, as the creeds tell us, "it's the same God." The assumption being that you can't use Jesus to say that God is always, unequivocally against war. Because, clearly, God isn't against war in the Old Testament. So God can't always be against war because, again, "it's the same God." To suggest otherwise is to flirt with the Marcion heresy.
So that's the argument. But I'd like to draw attention to the bait and switch going on.
Basically, the thing to note is this. The claim "it's the same God" is, as we've seen, a confessional rather than a hermeneutical assertion. More precisely, the confession "it's the same God"--"We believe in one God, the Father Almighty, Maker of heaven and earth"--hands us a hermeneutical conundrum.
The confessional statement "it's the same God" creates rather than solves the hermeneutical problem.
So the problem with the blogger I was reading was that he was using "it's the same God" hermeneutically rather than confessionally. In saying "it's the same God" he was, in effect, flattening out his hermeneutic so that the texts of the Old Testament could be read as equally important to the revelation of Jesus Christ. The blogger slid from confession to hermeneutic. That's the bait and switch.
To be sure, the distinction between confession and hermeneutic isn't as tidy as I've made it out here. But the point holds that the confessional statement "it's the same God" doesn't help resolve the hermeneutical issues. But what clearly shouldn't happen is deploying the confession "it's the same God" to flatten out your hermeneutic so that the war passages in the Old Testament are placed on equal footing with Jesus Christ.