The armies of heaven: at first glance it seems that these are angels; but the fact that they are dressed in fine linen, white and pure (see verse 8), indicates that these are the victorious martyrs (see similar descriptions in 3.5; 7.14). And in 17.14 the soldiers of the conquering Lamb are his faithful followers. Like their leader, they also ride white horses.

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on The Revelation to John. (UBS Handbook Series). New York: UBS, 1993. For this and other handbooks for translators see here .