And the ARMIES which are in HEAVEN,
This verse continues the military symbolism. The non-military form of this symbolism is found in Micah 5:2-5-6 where the 7 shepherds are thought to be Christ and the 8 leaders of men being the ARMIES. Note how the passage begins with Bethlehem and ends with Nimrod + Assyria (Babylon). Furthermore, 8 represents divine perfection as does the fact that those in the ARMIES are
clothed in fine linen, white and clean,
as represented in verse 8 also. See the description of the holy ones in Zechariah 14:1-5-9 and the unblamable in Holiness in 1 Thessalonians 3:11-13. It is clear that we are again seeing righteousness triumph over evil.
Interestingly, this army is not camouflaged to avoid detection as modern earth militia are. Rather this is the TRUTH coming forth boldly like a light set upon a hill (Matthew 5:13-16). There is no doubt that evil and those committed to evil (who took the mark of the beast and worshiped the image of the beast) can fully see Christ and His ARMIES coming. The conflict soon will be engaged (verses 17-21).
were following HIM on white HORSES.
All believers have followed HIM; that's how they got to HEAVEN in the first place, and thus are able to serve in the ARMIES. Using ARMIES (plural) implies that this is a collection of entities: Old Testament saints like those at the mount of transfiguration (Matthew 17:1-3-13) and the bones who came back to life representing the WHOLE house of Israel (Ezekiel 37:1-10-14), believers in Christ on WHITE HORSES, and angels (Matthew 16:21-27-28 and 2 Thessalonians 1:5-7-10).
We are not told what color the colt was, but it, too, was used in a triumphal entry (Luke 19:28-40). This is not a coincidence but rather a foretelling of this spiritual entry from HEAVEN into battle just as Christ went to battle to conquer death at Calvary.