What is the secret behind the courage of the Salaf that made them not fear lions and snakes?

Natural fear, such as the fear of lions, does not diminish the perfection of faith. An example of this is the fear of Moses and Aaron, peace be upon them. Sheikh Ibn Uthaymeen clarified that natural fear is permissible unless it leads to abandoning an obligation or committing a forbidden act, in which case it becomes forbidden. Allah may bestow upon some of His servants strength in their hearts, so they do not fear what is normally feared. Acquiring courage is achieved by attaching one's heart to Allah, knowing that what has befallen a servant was not meant to miss him, and what has missed him was not meant to befall him, perfecting reliance upon Allah, and filling the heart with fear of Him alone.