I'm always perplexed when people use the term "true Christian."
Christian means Christ-like. You either are, or you ain't. If you ain't, you are still headed to Hell. Sin doesn't send you to hell. Stopping sin doesn't make you avoid hell. It's a matter of belief. Do you believe the Gospel, that you are a sinner, Christ died for you, was buried, and rose again? If so, you are already a "true" Christian.
It is similar to when George H. W. Bush was asked, on 9-12-01, if he was a Christian. His answer was yes, but not a born-again Christian, like his son. There is no other way to become Christ-like. Jesus said, "Ye must be born again."
If you HAVE been born again, you are already a "true" Christian. That does not mean that you have received some special work of Grace and are now sinless or perfect. It simply means that your sins have been forgiven, and as in Scripture, when Jesus forgives your sins he says, "Go, and sin no more."
Why then do we choose to segregate Christian into categories? If we had to take a lie detector before coming in the church to decide who sits on what side of the building, I am afraid the building's foundation would be weakened by everyone being on the "not true" Christian side.
We all still sin, but thankfully we have an advocate with the father. 1 John 2:1
When someone points out your sin, or alleged sin, they are not saying that you are not a "true Christian." They are saying that you might be in sin. They recognize it, because they have been there. Maybe not in the same sin, but we all know what it is like how to fail our heavenly Father. They point it out for many reasons, beginning with your spiritual health. But it might be for the help of another.
To the sinning Christian (that's all of us), be forgiven (ask) and go and sin no more.
To the person segregating Christians into true and not true, stop. "Not true" Christians are simply Not Christians and need the Gospel.
