James 2:19
New American Standard Bible
19 You believe that God is one. You do well; the demons also believe, and shudder.
Matthew 7:21-23
New American Standard Bible
21 “Not everyone who says to Me, ‘Lord, Lord,’ will enter the kingdom of heaven, but he who does the will of My Father who is in heaven will enter.22 Many will say to Me on that day, ‘Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?’ 23 And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness.’
Those in the second set of verses not only believed in Jesus but they performed miracles in his name and were condemned.
This is why systemic theology is required. At first glance the verses seem to contradict one another. But in reality they don’t, they simply shed more light. So it seems, belief alone is not enough.