When I was a young lad, I had a friend whom we'll call Alex. I would frequently visit his house, and that included playing in his garden. One day, I noticed some plants growing. Previously, my mother had told me this species were weeds. Despite being weeds, they were rather beautiful flowers, but I had a rather seek-and-destroy attitude with weeds. If it was a weed, I believed it was a bad thing. So I thought I'd do Alex's mother a favor: I pulled the weeds.
In my young mind, I didn't understand why this got me in trouble. His mother came outside and told me to stop. "But mum says they're weeds!" I cried, pulling another from the ground. I was again asked to stop, but I didn't. As an adult, I now understand that other people's gardens are none of my business. They can plant what they want, trim what they want, and remove what they want. And I require permission to do any of that for them. The same applies to God.
In the parable of the tares, Jesus tells of a man who sowed good seed, but an enemy sowed bad seeds among them. When the bad seeds grew into tares, the man's servants asked if they should remove the tares, but the man said no, leave that to the reapers on the day of harvest.
Within the Church on earth is a mixture of true and false Christians. Some Christians are very obviously real, with solid doctrine, and it bears fruit in their lives. But sometimes it's not so obvious. Sometimes, people have not-so-good doctrine, and their fruits are minimal. Are they real Christians? Truth: You don't know. A Christian may appear to be a "tare", when in reality they're wheat. We can't all be expected to get saved and then instantly become John the Baptist. We grow at different rates, and we all have different strengths and weaknesses.
For that reason, although we are told to mark people who cause offences and divisions contrary to sound doctrine (Romans 16:17), we are limited in our ability to do so. "False Christian" is not a label we should use lightly. Rather, we should attempt to build up the Church, not letting any tares that do exist stunt our growth. False believers eventually reveal themselves anyway (1 John 2:19).