Ditto StoneyB. While his interpretation is clearly correct in this case, let me point out that this type of construction is potentially ambiguous. If, for example, someone wrote, "We should plan to get dinner and sleep on the train", that could mean that we should get dinner before we leave and then we should sleep on the train, or it could mean that we should get dinner on the train and sleep on the train. If I read this sentence without any further context, I'd probably understand it to mean that we would eat on the train. But consider the very similar sentence, "We should plan to pack and sleep on the train." We would probably understand that to mean that we should pack our suitcases before we leave, and then sleep on the train, not that we will pack while we are on the train.
In your example, the placement of the word "the" helps to resolve the ambiguity. If the writer had said, "Christians should think about humility, poverty, and the simplicity of the birth of Jesus", the placement of "the" on the third phrase indicates that it is separate from humility and poverty, so I would take that sentence to mean that we should think about humility in general, poverty in general, and then simplicity specifically as it relates to the birth of Jesus. (Such a sentence would make for a rather awkward thought, but that would be the strict reading.) By putting "the" in front of all three, then if we try to separate the first two from "the birth of Jesus", we are left saying we should consider "the humility" and "the poverty" and "the simplicity of the birth of Jesus". But what is "the humility"? It doesn't make much sense. So we're forced to associate it with "the birth of Jesus" to make a coherent sentence. This kind of careful wording is often used to force the right meaning on the reader. And while analyzing it here might sound complex, to the fluent speaker it often comes across quite naturally and easily.