Does it matter why someone believes?
If somebody believes the gospel, they are admitting they are a sinner and in need of a savior.
What if they believe but they are still completely wicked in their heart?
They won't. The action of the flesh is not the judge of the heart. Confession of the mouth is.
Will it be good to have them in heaven?
God thinks so. So much that he sent his son to die for those horrible sins in our place.