At the end of the parsha, things get worse for the yidden, and Moshe complained to HaShem about it. HaShem said to him, atah tir’eh, now you will see, but not in eretz Yisrael (Rashi).
The first idea here is that it is always darkest before the dawn. It was easy for them to believe in HaShem and Moshe as long as things went well. HaShem gave them a nisayon to test their emunah.
It says (and by now we know the song) that Moshiach does not come unless there is hesech hada’as. The song asks, how can we lose focus for Moshiach? I beleive the answer is based on what I wrote above. When Moshiach comes, it will be like in Mitzrayim. Things will seem great, but then turn for the worse, to test our emunah. This turn for the worse is the hesech hada’as, because it will not be on a silver platter to hope for him.
A second idea here is a limud zechus for Moshe Rabeinu. Think about it. Things were getting worse, and who knows exactly what would be. It seems to me that it may be worth it to so to speak “provoke” HaShem into getting agry at you for your lack of emunah, to thereby get an assurance from Him that all will suddenly become good. Atah tir’eh has two opposite meanings. One is that now you will see, but not later. On the other hand, now you will see (the miricles) is an assurance that all would now be well. Everything has a price. That assurance came with a price for Moshe. Perhaps it was worth it.