It does matter what we believe about the devil -- because if we don't think he exists, or that he's just a harmless cartoon character running around in a red suit with a pitchfork, then we won't take him seriously.
And that's exactly what he hopes we'll do, because then he can sneak up on us and deceive us with his lies and temptations, without us even being aware of it. Sometimes the devil is open and direct but often he is subtle and deceptive, and we can fall for his schemes without even being aware of it. We'll only make his work easier if we deny his existence or think he can't catch us in his snares.
You can be sure the devil exists, and that he is at work in our world, although often behind the scenes. How else can you explain all the evil and injustice in the world? How else can you explain the conflicts that tear apart our world -- and our lives? Jesus called Satan "a murderer from the beginning, not holding to the truth, for there is no truth in him.... He is a liar and the father of lies" (John 8:44).
But the greatest truth about Satan is that he is a defeated foe, for by His death and resurrection Jesus Christ broke the power of Satan. Don't be deceived, but put your faith and trust in Jesus Christ, who alone gives us victory over sin and death and Hell -- and Satan.