As Christian and Faithful journey through a wilderness, they catch up with Evangelist, who encourages them about the journey ahead. When Christian presses Evangelist for a more specific prophecy of what's to come, he obliges them with words of foreshadowing:
My Sons, you have heard, in the words of the truth of the Gospel, that you must through many tribulations enter into the Kingdom of Heaven. And again, that in every City bonds and afflictions abide in you; and therefore you cannot expect that you should go long on your Pilgrimage without them, in some sort or other. [...] [Y]ou will soon come into a Town [...] and in that Town you will be hardly beset with enemies [...] and be ye sure that one or both of you must seal the testimony which you hold, with blood; but be you faithful unto death, and the King will give you a Crown of life.
Evangelist's warning accomplishes a few different things. First, on a surface reading, it creates suspense, foreshadowing that Christian and Faithful will soon enter the city of Vanity Fair, where they will suffer at enemies' hands. Not only that, but either Christian, Faithful, or both will "seal [their] testimony [...] with blood," or die as martyrs. At this point, readers don't know which man's pilgrimage will end with martyrdom or if both will, and not knowing who will live or die—not to mention not knowing how it will happen, but knowing it will almost certainly be bloody—introduces tremendous tension, even though readers also know that martyrs are promised the "Crown of life" by Christ.
Second, on a deeper level, Evangelist's words foreshadow that every Christian should expect to suffer, as the Bible tells them that eternal life is only accessed by way of "many tribulations," "bonds and afflictions [...] in some sort or other." That might not mean that every Christian will face martyrdom or even great bodily or material suffering, but it does mean that no Christian should expect a perfectly smooth passage through life. Thus Evangelist's foreshadowing urges Christian readers both to heed the Bible's own warnings and to carefully watch what's about to happen to Christian and Faithful, gathering lessons to help them face their own afflictions in life, whatever their nature proves to be.
