Shemot (14: 1-5) (Robert Alter, trans.)
And the Lord spoke to Moses "... And I shall toughen Pharaoh's heart, and he will pursue them, that I may gain glory through Pharaoh and through all his force, and the Egyptians will know that I am the LORD." And thus they did so. And it was told to the king of Egypt that the people had fled, and Pharaoh and his servants had a change of heart about the people ...
Mishnah Bava Kamma 1:4 (Sefaria Community Translation)
The wolf, the lion, the bear, the leopard, the panther and the snake all rank as attested danger. Rabbi Eliezer says: When they are tame they are not ... attested danger, but the snake is always an attested danger.
Rabbi Eliyahu Dessler
Speaking to the Serpent in Gan Eden, God said, "You will attack him [the human being] at the heel." The meaning is that the Serpent -- who is the yetzer hara -- attacks a person at the point of his greatest weakness. This is the "heel" of the person: the sins which a person crushes with his heel. There are things in a person's life which he does not even call sins; he thinks them too small to concern him. But in fact, "small" sins repeated innumerable times may cause more spiritual damage to a person than greater sins, because he is aware of and concerned about the latter and will soon do teshuva for them, which is not the case with the "little" sins. It is therefore the unnoticed sins which constitute the person's lowest level.
Rabbi Shlomo Wolbe
We must always be on guard to ensure that [our] negative middos [traits] do not rear their ugly heads at a later point. ...
If people would pay more attention, they would notice that they have specific middos that surface regularly. It is these middos that are their battleground. The middos cannot be uprooted entirely, but if an effort is made, they can be restrained or even channeled positively.
Shemot (14-15: 30-1)
And the LORD on that day delivered Israel from the hand of Egypt, and Israel saw Egypt dead on the shore of the sea, and Israel saw the great hand that the LORD had performed against Egypt, and the people feared the LORD, and they trusted in the LORD and in Moses His servant. Then did Moses sing, and all the Israelites with him ...
they trusted in the LORD and in Moses His servant: The whole story had begun with Moses's understandable doubt to whether the people would trust, or believe, him. Now all doubt is banished (for the moment) in the great triumph at the Sea of Reeds.
Rabbi Henach Leibowitz
B'nei Yisrael would not have merited Hashem's Divine Presence and the gift of shirah, despite all the miracles that surrounded them, had they not worked on themselves to purify their hearts and acquire fear and belief in Hashem. The Midrash teaches us that we must exert ourselves to internalize the myriad of miracles constantly taking place around us -- from the perfect arrangement of the cosmos, to the incredible complexity of every cell in the body. If we work on ourselves, we can also reach lofty heights of fear and belief in Hashem.
Rabbi Shlomo Wolbe
How can I keep feelings of spiritual uplift from dying out? ... this high will inevitably die out by itself; these feelings are intangible, and it is impossible to hold onto them for extended periods. However, by transforming these feelings into action and giving them a practical expression, we can preserve our spiritual highs, because we will always retain the ability to hold onto the action it engendered.