| Quote #1
And three trees on the low sky,
The three trees, as crosses, signal the coming crucifixion of Jesus. The white horse is the apocalyptic white horse of conquest, which along with three other horses comes down from heaven to cleanse the earth of Satan and his minions. So while there is no overt death in the poem just yet, these lines are indicative of both mortality in the literal sense (bodily death) and in the metaphorical sense (conquest; death of a culture).
| Quote #2
And arrived at evening, not a moment too soon
Sure, these lines don't look like they have a whole lot to do with death or defeat on the surface. But it's an Eliot poem, so let's look closer. If we think about the ways in which "satisfactory" is used in the Anglican Articles, we see that this passage can be viewed as yet another reference to impending doom for the Magis' religion. Jesus was sent down as a mortal to satisfy the debt of the world's sins. Sins, it might be noted, that the Magis' culture and people were committing daily. More impending spiritual death.
| Quote #3
There was a Birth, certainly,
Here's where things take a turn for the literal. In this chilling passage, the Magus expresses his certainty that Jesus was definitely born, but he then turns right around and conflates Jesus's birth with death itself. He doesn't come right out and say it, though the implication of "had thought they were different" is that upon witnessing the birth of Jesus, the Magus realized that he was also witnessing the death of his own ways, and, if we stretch, his own physical death to come. Yeah, that's not the version of the Nativity we're used to.