Hashem says to them: Place your trust in My Name and it will save you, as the pasuk states: He shall trust in the Name of Hashem (Yeshayah 50). And why? For everyone who places his trust in My Name, I save him. You should know this from the fact that Chananya, Mishael, and Azaryah trusted in My Name and I saved them.
(Medrash Socher Tov 31)
What is the proof that everyone who places his trust in Hashem will be saved? According to the medrash the proof is that one time when three of the greatest Tzadikim who ever lived placed their trust in Hashem, as they were being thrown in a fire for not bowing down to an idol, Hashem saved them. How, at all, does this prove that everyone who places his trust in Hashem will be saved. Perhaps you will only be saved if you are a great tzadik?
Perhaps we can suggest that the fact that everyone who places his trust in Hashem will be saved needs no proof. It is simple to understand that a son who truly places his trust in a Father, who is all powerful and overflowing with a constant burning love for him, will be saved. The only doubt Chazal had was that perhaps since Hashem runs the world al pi derech hateva – in a natural manner, so that man will retain his free will, bitachon may not be possible when one can only be saved through a clear miracle. To negate this suggestion, all that had to be proven was that Hashem performed a clear miracle in order to save Chananya, Mishael, and Azaryah due to their trust in Him. Once we know bitachon is possible even where a miracle is needed, we fall back on our original assumption that, without a doubt, our Father-in-Heaven will never less us down.
Other parts to this series: Inspiration in Bitachon