Jesus explained the difference in Matthew 7:15-23.
"Watch out for false prophets. They come to you in sheep's clothing [i.e. appearing to belong to Christ as one of His 'sheep']… By their fruit you will recognise them...
"Not everyone who says to me 'Lord, Lord' will enter the kingdom of heaven, but only he who does the will of my Father who is in heaven. Many will say to me on that day, 'Lord, Lord, did we not prophesy in your name, and in your name drive out demons and perform many miracles?'
"Then I will say to them plainly, 'I never knew you. Away from me, you evildoers!"
Those, plainly, are not true Christians, but hypocrites who speak and do wonderful things in Christ's name, but He will disown them. It is what Jesus says on the subject and His judgment that I go by, because I'm a Christian.