And why at any rate would the German Tribes convert to Christianism? I mean they are Tribal Federations with a Warrior Culture. Why would they adopt a Belief preaching love and peace? The argument that Romans threatened them with Military Might is here invalid since they never managed to take them down.
And it was a set-back for Christianity. However, Christianity had already gained luster as the belief system of those ever-so civilised Romans, and most Barbarian tribes were anything but proud of their own traditions from the heaths and hinterlands of the world. When Clovis, King of the Franks, converted to Christianity at his wife's urging, he had already mostly shed the traditional Germanic gods and was worshipping the more prestigious Roman pagan gods, possibly with a syncretic mixture of Germanic beliefs thrown into the pot.
The Visigoths likewise were already Arian Christians before they were allowed to cross the Danube and settle in Hadrianopolis. They retained a great deal of the Germanic heroic and warrior codes that lent them ferocity in battle, but the process of becoming christian had already been started on a superficial level and would develop further. Sacking and burning Rome was merely a bump on the road toward that ultimate Christian destiny. In the end, most barbarians didn't want to destroy Rome or its institutions, they just wanted to be Rome and its institutions... and were really,
really bad at getting it right. They just wanted to pet the rabbits, George!
Also, although the Germanics had war-like cultures and traditions, the Jesus that was sold to the Frankish nobles was much less pacificist than perhaps the version sold to their wives, and to their Gallo-Roman serfs. He was more of a "just lord" presiding over his comitatus of faithful followers in a way that any German could understand, and who generously gave wealth and prosperity to loyal followers in the germanic tradition of a breaker-of-rings. When Clovis was told the story of the crucifixion, he reportedly blurted out, "If only I had been there at the head of my Franks!" which obviously ignores the point of the crucifixion as an intended sacrifice--the Germanics were eager to be Christian and assume the trappings of civilisation, but still remained deeply invested in their warrior traditions.
As for how the Germanic warrior code meshed with Love Thy Neighbor, they generally didn't absorb that part well. But, then again, who really does?
One factor I'd like to point out is that Christianity was very rigid and stiffly defined compared to earlier religions, particularly after the Romans edited the Bible down to a simple canon. So "pagan" beliefs would often mix (you can find combinations of Roman and local gods in all sorts of places, for instance, or people who worshipped both Roman and local deities) and dilute, while Christian belief could spread wholesale and displace whatever religion had been there previously.
Jesus didn't say: "Yea, verily, if ye love the Lord thy saviour, ye shall raise up a lighted pine tree on my birthday and give gifts to each other. Likewise, on the date of my death, yea verily, celebrate my love of rabbits and eggs."
However, Christians get it from both ends. Either they're too rigid to accept parts of other beliefs, or they wickedly stole parts of other beliefs. Either way, they suffer criticism. In reality, while it's definitely true that Christianity was more organized and unified than some other religions, it's also true that it borrowed bits and pieces of other beliefs and not in the mean-spirited and conniving way that is often suggested. Syncreticism is natural whenever religious beliefs overlap.
They weren't any more warlike any other culture. Particularly not Roman.
I believe I could argue otherwise. Tacitus' Germania already suggests so. The tradition of Germans going to their Things with their spears to show their eligibility to vote compares to the more placid voting of Greeks and Romans as somewhat warlike!