I am not endorsing or promoting Francis Turretin as a true Christian when I quote from him. But I think he puts forth some interesting information.
“XIX. Although the pure preaching of the word does not always prevail in the church, it does not follow that this mark is separable from the church and that it is therefore falsely said to be a mark. That purity ought to be understood with a certain latitude, nor does the church at once cease when the purity ceases according to some degrees, provided it does not cease altogether. Purity ought to be in fundamentals in order that it may be a true church, although in other respects various errors can obtain in it from which it could contract various degrees of impurity (which although they take away from it the name of a pure church, still they do not remove the name of a true church, as long as the foundation remains safe and unimpaired). The pure preaching of the word and the purity of the church walk hand in hand. If the former is in every part pure and free from error, the latter also will be pure; but if the church begins to be corrupt it does not at once cease to be a true church until the foundation is assailed” (Turretin; underlining mine–CD).
To focus on the underlined portions. What does it MEAN to preach purity in the fundamentals? How does one preach in order that “the foundation remains safe and unimpaired”? Answer: They preach THE GOSPEL of the PERSON and WORK of JESUS CHRIST.
What of a true assembly that “[contracts] various degrees of impurity” that would remove “from it the name of a pure church”? Well, we see how the apostle Paul dealt with an assembly of professing Christians in Corinth who were proud or “puffed up” with apparent tolerance and open-mindedness (see 1 Corinthians 5:1-8).
Clearly (obviously), one must define precisely WHAT THEY MEAN by “various degrees of impurity,” and also provide examples of HOW a “true-yet-impure” assembly might appear in view of Scripture. 1 Corinthians 5:1-8 (especially verses 7-8), shows Paul engaging in something quite strange, peculiar, and perhaps even downright surly and mean. Something called — gasp! — church discipline.
A large and significant portion of “professing Christian churches” who believe that Jesus Christ died for everyone without exception (i.e., daughters of the whore) do NOT even pretend to exercise discipline regarding doctrine and obedience (character and conduct). And really, WHY WOULD THEY discipline over such trivial matters as spiritual and physical adultery? They are, after all, daughters of the whore. Like mother, like daughter.
“Behold, all who use proverbs shall use [this] proverb against you, saying, As the mother, so is the daughter” (Ezekiel 16:44).