Revelation

Another piece of all this is fitting this horse and rider in with the other four, and fitting it in with the first four trumpets and the first four bowls (here’s where 5 down has to agree with 3 across in my crossword illustration). 

Fitting it with the other horses really works well either way.
If the white horse and rider are the Gospel riding forth then the other three are natural progressions that fall in the wake of such a movement, war, pestilence, and death.  On the other hand if the white horse and rider are antichrist figures, false teachers, and false religions, then horse #1 fits very well with horse #2, #3, and #4 as they would all come from the general group of major opposing forces to the church. 

Fitting it with the 1st Trumpet, and the 1st Bowl is a bit more tricky.  The 1st Trumpet and the 1st Bowl both deal with destruction happening to the earth.  If the trumpets serve to warn those who have either consciously or unconsciously squared themselves off with God and the bowls are clearly God’s wrath being dished out on those who have consciously and willingly taken the opposing side, then the 1st seal, which is the 1st horse would best fit representing a force that has placed itself against God. 

Also there is that element of “the earth.”  The beast of the earth spoken of in Revelation 13 is described elsewhere in Revelation as the “false prophet” (16:13; 19:20; 20:10).  The beast of the sea represents governments, kingdoms, earthly rulers, and the sort.  If the earth is the symbol for the false prophet, and the earth is the key element referred to in the 1st Trumpet and the 1st Bowl and these are to line up with the 1st Seal then the white horse and rider would fit well as antichrist figures, false religion, and false teachers through history.