Is intense fear of the horrors of the Day of Resurrection and the torment of the grave, and the feeling that one's deeds are insufficient, considered a weakness of faith? And how can one's heart be made firm?

The Muslim's fear upon remembering the horrors of the Day of Judgment and the torment of the grave is a sign of goodness and an indication of the soundness of his faith. This should be followed by striving in obedience, preparing for death, and working to escape those horrors. Among the most important means of remaining steadfast in religion are seeking help from Allah the Almighty, striving in supplicating Him, struggling against one's own self, accompanying the righteous, adhering to the remembrance of Allah the Almighty, and frequently contemplating His Names and Attributes.