It is stated at the beginning of Parashat Beshalach (Exodus 13:19), "And Moses took the bones of Joseph with him, for he had made the children of Israel swear, saying: God will surely remember you, and you shall carry up my bones from here with you." It is brought in the name of Rabbi Yosef Dov HaLevi Soloveitchik zt”l, to question why this matter is mentioned here and not in Parashat Bo in the story of the Exodus of the children of Israel from Egypt, where it seemingly belongs, as when they left Egypt, Moses took Joseph's bones with him. Rabbi Yosef Dov explained that this verse continues from what is said above (Exodus 13:18), "And the children of Israel went up armed out of the land of Egypt," which Rashi interpreted as "armed with weapons." When they left Egypt, the children of Israel took with them weapons for war, and as a continuation of this, the verse says, "And Moses took the bones of Joseph with him," to imply that the weapon Moses took was Joseph’s casket.

As found in the Midrash (Yalkut Shimoni on Psalms, Remez 833), on the verse (Psalms 114:3) "The sea saw it and fled": it saw Joseph's casket descending into the sea, and the Holy One, blessed be He, said: it shall flee because of 'the one who fled,' as it is said (Genesis 39:12), "And he left his garment in her hand and fled, and got out." Thus, the most effective weapon they needed was Joseph’s casket, and Moses knew this, so when everyone armed themselves with weapons, he 'armed' himself with Joseph's casket, and that is why this verse is mentioned here and not in Parashat Bo.

According to our approach, the foundation of the miracle of the splitting of the Red Sea is the concept of trust (bitachon). To merit this miracle, they needed the merit of trust. It is well understood from where Moses knew that Joseph's casket was the best weapon to cause the sea to split – because Joseph was perfect in this trait of trust. As we have discussed before, the root of Joseph's strength in overcoming desire and temptation in the incident with Potiphar’s wife was that he did not lose hope and did not despair of his situation. Instead, he stood firm in his faith and trust in God. If he had despaired and sunk into a state of gloom, it would have been impossible for him not to fail and fall into the temptation. Therefore, we find that God was very meticulous with him regarding this matter of trust, as brought in the Midrash (Bereishit Rabbah 89:3): "Blessed is the man who makes the Lord his trust" (Psalms 40:5), this is Joseph, "and does not turn to the arrogant" (ibid.), because he said to the chief butler 'remember me' (Genesis 40:14) and 'mention me' (ibid.), two additional years were added to him. Such meticulousness, due to two expressions of 'remembrance' in his request to the chief butler to mention him before Pharaoh – 'remember me' and 'mention me' – caused him to be confined for an additional two years in prison. Such meticulousness is found only with a perfectly righteous person whom the Holy One, blessed be He, is meticulous with like a hair’s breadth. And Moses, knowing this perfection of Joseph in the trait of trust, recognized and understood that his casket was the best weapon to 'arm' himself with, so that when the sea saw the 'one who fled' in his great trust, it would flee before him.

This adds another depth to the words of the Sages in the Baraita (Sotah 13a): "The Rabbis taught, come and see how beloved are the commandments to Moses our teacher. While all Israel was engaged in plundering, he was engaged in the commandments, as it is said (Proverbs 10:8), ‘The wise of heart will take commandments.'” According to our approach, we can deepen this and say that not only because of his pursuit of commandments and taking them did the Sages call Moses 'wise,' but also because of his profound understanding, that he was wise enough to understand the power of Joseph's casket and the benefit that could come from it for the splitting of the Red Sea.