God protected Joseph Smith until his earthly mission was complete.
The first section in the “Teachings” portion of the lesson is primarily a short recapitulation of the previous lesson, which focused on Joseph’s understanding of his own mission (and so included a number of teachings in which Joseph asserted that he would not be taken before he had finished his mission). But it also makes a few nice points that deserve attention.
In the first paragraph on page 531, Joseph effectively introduces the theme of the subsequent section of the lesson: “my mission in this life” is not only to establish the Kingdom, but to establish it “so firmly . . . that all the powers of earth and hell can never prevail against it.” I’ll have much more to say about this theme in the next section.
In the fourth paragraph on the same page, Joseph ties the question of truth to the question of exaltation: “he that is afraid to die for the truth, will lose eternal life. Hold out to the end, and we shall be resurrected and become like Gods, and reign in celestial kingdoms, principalities, and eternal dominions.” I like this. It makes clear a consistent theme in the Book of Mormon—namely, that salvation (or exaltation) is a question of one’s relationship to death. Joseph nicely adds to that theme, though, the fact that what must determine one’s relationship to death is, specifically, truth. This has, of course, been a relatively consistent theme throughout the manual, because Joseph has a good deal to say about truth.