Christianity had already started to penetrate the Amazigh tribes living in the Northern Sahara-what is now southern Algeria, Tunisia, and Libya.
Actually, if we look after the clearly identified Christian Berber tribes and kingdoms, it's clearly limited to former Roman sphere of influence or provinces (the mentions are almost always limited to the coast), while Judaism clearly overshadow them as for importance in the same region.
Mauretanian and Saharian Berbers were probably touched only partially by that, if at all.
Give it enough time and develop the trade routes some more, and it could easily spread south as it did along the Silk Road, hopping from oasis to oasis.
Thing is, the SIlk Road was huge, ancient and dynamic. You didn't have an important trade road between Mediterrenean Basin and Sub-Sahelian Africa, let alone something comparable.
Trades roads doesn't randomly pop out : you need an impetus. Slavery was the main one IOTL, but considering its situation in Late Romania, it have to be ruled out.
And without this trade, and the constant raids in the region, you have less knowledge of the aera and its possible ressources.
Another problem is that you need relativly stable polities to provoke a stable trade road : OTL, Arabo-Islamized states (as Ghana or Mali) that entered in relations with the cultural-economical AI continuum (more or less forced by their neighbours, or to prevent being targets of slavers by converting).
Of course, it's not butterflied there, as entites as the "proto-Ghana" began to appear, and city-states/tribal-states of Somalia already existed. But the process would be longer and smaller than IOTL Islamization (and may require Christianisation of Arabia)
There are poor souls down there who haven't had the opportunity to save themselves, let's send them missionaries.
Ancient and Early Medieval Christianisation doesn't really work that way.
It's above all things a diplomatic feature : you do have missionaries on the loose, but generally unsucessful if they're not backed by a patron (as Byzantium for Russia), and it may even require pure and simple conquest of pagan tribes or polities to enforce it.
See, it's important for already Christian entities that the right king of Christianity is teached (as in orthodoxy), and critically under their political control : as Christianisation was more or less the era's equivalence of a diplomatic recognition (as in part of UN nowadays), it wasn't to be toyed about.
You then need an impetus for christian entities to enter in relation with sub-sahelian african entities, relations going from alliance to full war.
Problem : without booming trade, these african entities would have trouble to devellop out of nowhere, and would be kind of isolationists if develloping into isolation.
It's not an obstacle (see above) but would be more tricky to manage.
And while they are there, they might also report back as to any interesting trade items, even if for the time being we are not seeking for any.
It's partially true.
Partially because a lot of ressources of sub-sahelian africa were known after slavery trade began. Salt and Gold trade, by exemple, were seen as compensation to still have an economic relation without having your villages raided every two or four years.
As we're talking of entities develloping on themselves, they simply might not see a great interest develloping these ressources and their trades at a noticable scale.