What does it mean, “He...ascended far above all the heavens, that he might fill all things?”

Well, for starters, what you heard in Mark 16:20 begins to answer the question. “And they went out and preached everywhere, while the Lord worked with them and confirmed the message by accompanying signs.” As you heard, this was stated following the Lord’s ascent from the disciples eyes. Out of view, Jesus is now bodily present everywhere. Don’t ask me how this works; this is simply the truth that is confessed according to the Scriptures and espoused in the Lutheran Confessions. In His bodily omnipresence, Jesus fills all things. I suppose that were He still manifested on earth, it would be more difficult to grasp His omnipresence—you could point to a person and say, “Jesus is there,” and the eyes of fallen flesh would or could override what your eyes of flesh “see.” Out of sight, you can believe, though not fully understand, that Jesus is everywhere, filling all things.