More than likely the word derives from the old English faken generally meaning fraud.source
When Elymas the sorcerer tried to dissuade a proconsul named Sergius Paulus from becoming a Christian Paul the apostle turned and stared him down. “you who are full of all deceit and fraud, you son of the devil, you enemy of all righteousness, will you not cease to make crooked the straight ways of the Lord?” (Acts 13:5)
There are few things in scripture worse than being a fake. Littered throughout the pages we see fake prophets who speak what God has not spoken. Their penalty was stoning. Fake teachers who teach what God does not teach. Their end is destruction. And fake Christians who either through ignorance or arrogance do not have an authentic saving faith in Jesus. It is this last group which should cause us the most consternation. Their end is unexpected.
On the last day when God has separated the sheep from the goats, the ones on his left will say, “Hey wait a minute. We preached great messages and we cast out demons. We spent a good amount of time doing all kinds of service for you!” But Jesus will respond, “I never knew you.”
How terrible to get to that last day and discover that you were after all a fake. But we do not have to guess. Paul instructed the Corinthians to “test and see” if they were in the faith. (2 Corinthians 13:5) The implication above others is that we can indeed know that we belong to God.