Mark 9:
38 “Teacher,” said John, “we saw someone driving out demons in your name and we told him to stop, because he was not one of us.”
39 “Do not stop him,” Jesus said. “For no one who does a miracle in my name can in the next moment say anything bad about me, 40 for whoever is not against us is for us. 41 Truly I tell you, anyone who gives you a cup of water in my name because you belong to the Messiah will certainly not lose their reward.
That is an interesting verse.. you posted.
Its shows the distinction. between those who knew and know Christ, and were a part of Him, yet were not "of the original 12". Yet, there are those who will say to God in Rev. 20:11, "have we not cast out demons and done works in Jesus's name"... And they will be told to DEPART = I never knew you.
So, its possible to perform and behave as a Christian, and sound like one, and pretend to be one on a Forum, or in a Pulpit, and yet, Hell is your home in the end because you were never truly born again.
This is something to consider, because, Satan is real. His people are often fruit bearing types who can quote verses and seem like they are really Christians.
"Satan comes as an angel of LIGHT"......and so do his deceived and not deceived.
The way you spot them, is to notice how they deal with the Cross and the Blood of Jesus and the Blood Atonement.
They will always refuse it, and replace it with "enduring", or "Abiding" or "self effort to stay saved", or the Torah, or some type of Obedience....etc.
Satan's workers, will always replace GRACE with SELF EFFORT.
ALWAYS. !
Learn to "hear" them. Learn to read them in text. They always "out" themselves by how they mistreat the Grace of God and deny and replace the Cross with some type of "self effort" as their Salvation, as their gospel of works.
This can be commandment keeping, or "enduring to the end" as their gospel, or 'Abiding" as their gospel of works, that they teach is how you keep yourself saved.
See, the devil always wants you to be your own savior by your self effort, as that is how he replaces the Cross and the Grace of God.
When you see-read these people teach this, you have seem them as they are, and this is always as a dark light, and never as the Gospel Truth.