This is a broad generalization and is probably wrong, but, the way I see it, Christianity hasn't been very succesfull in converting horse-riding nomads.
Christiany had a head start in North Africa and the Middle East, but, although some Berbers and some Arabs had become Christians before the VII century, the truth is that the vast majority of the population of the Sahara and the non-sedentary population of the Arabian peninsula were Pagans, not Christians (a few Arabs clans were Jews, IIRC).
Christianity had also a head start in Central Asia, but, although some groups converted to Nestorianisms, there weren't massive conversions, and, eventually, most of them become Muslims,...or (later) Budhists.
Finaly, despite the might of Constantinople and the atraction it exerted, nomads in what's now Ukraine or the Russians stepps rarely converted to Christianity. Some did, but only after they had settled in the Balkans, like the Bulgars (their relatives in the Volga bassin converted to Islam) or in Hungary (the Magyars).
Is there a reason why Christianity succeded more with sedentary societies like pre-islamic Egypt than with nomads like the Berbers? If so, why did it happened, and how could that have been changed???