If the enemy can get you to believing your prayers are prayed in vain, you will stop praying. If he can convince you, there is no heaven and no hell, he can steal your life, kill your hopes, and destroy your future.
Why don’t you just pray anyway and believe anyway? Because you won’t get an answer if you don’t ask, and you won’t know if hell exists until you get there. And then, it’ll be more real than ever, and you won’t get to change your mind. God wins you by love. But the fear of God is the beginning of all knowledge.