Some nice comments Jane. The mighty sword (with cross in tow) of Charles the Great (Charlemagne) was indeed one of the key reasons the various tribes did not unify somewhat earlier and were Christianized to the last, but it has to be remembered too that they were never a wholly united front, but instead a confederation of racially and culturally related peoples (tribes), who shared the same tongue, and thus could communicate for the most part freely. But we are still talking of huge numbers of peoples, and even before the full weight and impact of the Greco-Roman civilization hit them, when they had yet to cross the Danube en masse, there were already considerable differences manifest between the leaders of these various tribes. This most certainly is reflective of the differences that existed culturally between the various clans even thousands of years ago, some being rather minor, others being more significant. It is believed that even back then there were considerable linguistic differences (idiomatic variations) from the westernmost and easternmost tribes. It would be quite a stretch to think that tribal clans as diverse as the Alemanni, Teutons and Suevi (present day Swabians) all saw eye to eye across the board, or mimicked each other in all their customs.
Moreover these massive groups did not all congregate at the door of Europe at once. It took hundreds of years for them all to eventually depart the Russian plains and steppes in their massive western movement, and the great Scandinavian peninsulas southward. For almost two centuries Germania consisted of territories to the east of the Danube, and by the time they finally breached the Rhine, Rome and Gaul's days were numbered.
Remember too that their explosion upon the Roman world had probably less to do with lure or outright avarice (the opportunity of taking advantage of a weakened and divided Roman world before them) than it had to do with the pressures from the Huns and Slavs agitating to their East, who were wreaking havoc upon those tribes furthest east (the Ostragothic clans, et al) in their own push to expand westward. For hundreds of years the Germanic tribes had been interacting to one extent or another with the Romans and their faltering empire, often skirmishing or warring with it. Indeed because of their warlike tendencies and prowess, they were recruited (and/or enslaved) in large numbers to serve as auxiliary troops for Roman legions (Goth mercenaries, and Romanized Germans starting with Caesar), and thus many a Roman-German battle of yore involved Germans fighting Germans (and in large numbers at that). This became more and more pronounced as the might of Rome slipped further and further into decay. It is widely believed that by the end of the Empire, there were more Goth troops in Roman uniforms that Italian, and this eventually came to include much of the officer class itself.
One other point that should be made, in the name of accuracy. Most of the Germanic tribes never bent to the will of Rome (nor to Slav or Hun influences either), nor did most adopt Roman ways after they conquered or occupied Roman territories. Obviously there would be some influence carried over practically everywhere they settled, as the Roman ways were considerably more advanced, but for the most part they remained quite heathen-like and content in practicing their Germanic ways of old. The eventual power of the cross, carried to so many at the point of the sword, had more to do with this transformation (the synthesis of German to the Greco-Roman) than the sacking of Rome itself, which only a small percentage of tribes, mostly Gothic, actually participated in (there was also the considerable cultural influences from the Celts and Nordic Gauls as they were conquered or absorbed). Needless to say, most of the Germanic peoples went right on speaking Deutsch and practicing their German culture of the past, whereas only the westernmost tribes adopted (assimilated) more fully the Roman culture and even tongue (thus my referencing the Franks, Visigoths, Lombards, Vandals etc.). It may have been (though open to debate) one of the very reasons Charlemagne had a leg up on his fellow Germans back then, and could wield the sword and cross so effectively upon them.
The subject of the Saxons is one of the most compelling and complex, and could fill volumes alone in its dissection. The English we speak, though Germanic at its base and core (after all Vater is still Father and Mutter = Mother, Bruder = Brother, Schwester = Sister etc.), has evolved into such a polyglot of diverse contributions and injections that it is a marvel unto itself. The synthesis over the centuries of so many varied idioms, from Roman and Greek to Norman, Gaelic, Scandinavian, even remnants of early Breton etc. have completely transformed it to where, as we all know, the Saxons in Germany have no idea what their distant Saxon cousins there in old Angleland (er, England) today speak, and vice versa. We in America do, but that's of course due to the fact that the Anglo-Saxons were never really content just to remain put and cultivate the pleasant soil of England, but had once again set their sights on western exploration and expansion, much like their ancient kin did.