I suggest you not call the trees in your orchard rotten, just because the fruit are different. Different doesn't mean rotten.
I understand your point of view.
I do not have a problem if persons want to accept anyone who claim to be a Christian, to be such. We all have our choices to make.
However, If I were hiring an acrobat, or juggler for my circus, and they couldn't perform, they wouldn't be joining my circus.
Another person may not mind the imitations, but they have to decide what they want their circus to be like.
Since you mentioned denominations or religions, then that is the way I look at it.
The disciples were called Christians. They were known as those who followed Jesus, by what they taught, and how they lived.
Festus could tell the difference.
Today, I see it the same way.
Either a religion is following Jesus, its teachings and practices, or it is not following Jesus.
Individuals, in that religion will either practice what they preach or not, but I am not the one to judge them.
However, as with first century Christians, there are measures pu in place, to keep the Christian congregation clean.
1 Corinthians 5:9-13
This way, it becomes possible for people with honest heart to find the way to God. Those who are not really honest hearted, will remain in the dark - confused. 2 Thessalonians 2:8-12
You have still not answered the question of how to determine if a person is “actually” a Christian. Everything in the Bible has been interpreted and cherry picked into oblivion. There is no objective standard