As regards the religious environment of Late Antique Arabia:
"By the early seventh century it is possible to speak of a fledgling Arab Christianity, based in the settlements of Rusafa (in northern Syria), Hira (southern Iraq), Najran (northern Yemen), and a number of places in the Roman province of Arabia stretching from Jabiya in the north, in modern southwest Syria, to Petra and Kilwa in the south, in modern south Jordan and northwest Saudi Arabia, respectively."
In God's path: The Arab conquests - Robert Hoyland
At Kilwa they found the following:
The date of occupation of this monastery will remain undetermined until the final outcome of the studies of the excavated materials is known. In the meantime, we might base our assumptions on the Arabic inscription found on the lintel of cell 63 (Fig. 16, photograph and facsimile)3 which translates as follows:
Bism Allâh ḥimat ‘hl Taklâ min ‘Iqlîm
In the name of God, this is ‘the forbidden land, the irrigated field’, belonging to the community of Takla, originally from Iqlīm.
The first remark we would like to make concerns the script. It is identical to the inscription at Qaṣr Burquʿ, dated 81 AH/AD 700 by the Omayyad caliph, al-Walīd, before his ascension to the throne (AD 705–715). According to Savignac’s observations, the lettering of the text resembles script dating from about AD 1000 (Horsfield, Horsfield & Glueck 1933: 381). Unlike the text of Qasr Burqu’,
we owe this text to Christians: the name of the saint is clear from the reading (Takla). The name seems to refer to Thecla (Takla), the famous saint whose cult flourished in Seleucia and Syria during the fourth and fifth centuries (Davis 2001)... The monastery of Kilwa, which still retains much of its mystery, may very well help us to gain a greater understanding of the expansion of Islam and its behaviour towards other communities, in this case Christianity. It is essential to stress that the crosses in Kilwa, more than ten in number, are all in very good condition, as are those in Qaṣr Burquʿ (Gaube 1974: 98; Field 1960), and that this Christian symbolism has never been damaged. Further work on Kilwa will improve our knowledge of the historical geography of Arabia at the beginning of Islam, and will help us to understand the spatial organisation adopted by nomadic peoples in arid areas.
Christian monasticism on the eve of Islam: Kilwa (Saudi Arabia) — new evidence -
http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0471.2011.00335.x/abstract
None of this on its own really 'shows' a great deal. Just more examples of how the region wasn't this pagan backwater completely separated from the rest of the region. The people of the region were not ignorant of monotheism and the contemporary religious issues of the wider region.