As a body of Christ, there is only one invisible church, while there are plentiful visible ones in this world. Therefore, it is necessary for believers to discern where the line is drawn. Because the true body of Christ is an invisible one, the only means to be part of her is through the visible ones. It stands to reason that no visible church is ever perfect, as the invisible church is.
It should already be understood that there are fake believers attending churches. There are those who don't even realise they are phonies. The best of them have loyalty, commitment, diligence, but they lack love, faithfulness and faith. These are not those who denounce Christianity, they are not those who said there is no god. They may not even be those Mormons or Jehovah's Witnesses who firmly believe they are Christians but pretend they don't know every other Christians are wrong or fake. No, that would have been too easy to identify and be treated according to biblical church discipline. I am talking about those who attend church for the sake of being good. Those who sing hymns with others from service to service without any relation to God. Those who pray, if they do pray, but it is always uni-directional, so that their prayer is always requesting something from God. Those who dare to use the term "brothers and sisters in Christ" but hope to stay away from any believers they do not like. These phonies, are whom I am describing in this paragraph. Why fake? Not because they've failed as I have described above, no, not at all, for God is merciful. But fake because they make the failures above their lifestyle, their behavior, even their principle! These, are the ones who are in the visible churches but are definitely not in The Invisible Church.
Our Lord Jesus has touched on this subject many times. The comparison between goats and sheep, the parable of the tares, the parable of the sower, etc.
Though the problem of the visible churches is inevitable, we see a constant ignorance towards the warning. I find two causes: 1. The injection of false teachers, 2. The need for social connection surpasses a worship that is in truth.
Despite the development of education we have today, false teaching are still well accepted in churches today, if not more than the past. For example, the parable of the sower refers true believers as the good soil only. But it has been widely taught that