The one point of religious law that attaches specifically to men's robes concerns length. A group of hadith warn against letting the lower garment trail below the ankles, and the usual explanation given in the reports themselves is that trailing a garment was a display of wealth and arrogance. From this, Salafi practice generally holds that a man's thobe should end above the ankle, and the shortened hem is accordingly a visible marker of religious orientation across the Gulf, South Asia and the diaspora. Other scholars read the prohibition as attaching to the arrogance rather than to the cloth, so that length is only an issue when it is a display, and men in the same mosque may wear both.
Beyond that, the thobe is regional dress. The cuts are national and are read as such. The Saudi thobe has a shirt collar and cuffs; the Emirati kandura has a round collar with no buttons and a tarboosh, a tassel at the neck; the Omani dishdasha has a distinctive collar and its own tassel; the Kuwaiti and Qatari forms differ again in collar and pocket. Colour follows season and occasion, white in summer and darker shades in winter.
The headdress worn with it, the ghutra or shemagh held by the black agal cord, varies the same way, and the red and white check associated with the Levant and the plain white of the Gulf are markers of origin rather than of piety.
Two things follow. Arab Christians wear the thobe, because it is the dress of the region and not of a religion. And the modest dress requirements of Islam apply to men in specific and enforceable terms, a point that Western discussion of Islamic dress consistently omits.