Why were they able to cast demons out? The seven sons of Sceva, who weren't believers, weren't. What made these men able to use the Name of Jesus? They were believers.
In Matthew 10:1, we see that Jesus gave His 12 disciples power over unclean spirits, to cast them out, and to heal all kinds of sickness and all kinds of disease. This would include Judas Iscariot, yet Jesus referred to Judas Iscariot as an
unbelieving, unclean devil who would betray Him. (John 6:71; 13:10-11) Apparently, Judas believed that Jesus' name has the power to cast out demons but did not truly believe in His name (John 1:12) and become a child of God, but instead was the son of perdition. (John 17:12)
Ezekiel says "if a righteous person turns from his righteousness... his righteousness will not be remembered"--hence statements like "I never knew you" (which is the language used in blotting someone out, the opposite of "confess before My Father"--ie, "deny before My Father") and "they were never of us". You have to understand Scripture through Scripture not import your beliefs into Scripture. When someone is blotted out it is as if he were never there in the first place.
Ezekiel 18:29 - But the house of Israel says, 'The way of the Lord is not right.' Are My ways not right, O house of Israel? Is it not your ways that are not right? But the house of Israel says, 'The way of the Lord is not right.' Are My ways not right, O house of Israel? Is it not your ways that are not right? Ezekiel 33:13 - When I say to the righteous that he shall surely live, but he
trusts in his OWN righteousness and commits iniquity, none of his righteous works shall be remembered; but because of the iniquity that he has committed, he shall die. Did you read that? "If he
trusts in his OWN RIGHTEOUSNESS." This is the heart of the matter here. This is describing a
righteousness which is by the law rather than that which is by faith. The New Testament states in Romans 10:3 - "For they being ignorant of God's righteousness, and going about to
establish their OWN RIGHTEOUSNESS, have not submitted themselves unto the righteousness of God. 4 For
Christ is the end of the law for righteousness to every one that BELIEVES.