I am NOT endorsing or promoting Francis Turretin as a true Christian when I quote from him, nor any other professing Christian I might use to make a point (e.g., Athanasius). Turretin writes:
XVII. It is one thing to ascribe the marks of the church falsely to themselves and to boast of them; another to possess them truly. That is of fact; this is of right. The false boasting of heretics claiming the marks for their assembly ought not to prejudice the certain persuasion of believers because we must judge of marks not from the dreams of the sick or the opinion of the proud, but from the truth of the thing.”
The “truth of the thing,” he says. Correct. Not mere PROFESSION. But actual POSSESSION. Many PROFESS that the true gospel wherein the righteousness of God is revealed is CENTRAL and ESSENTIAL. But when you discover who they acknowledge as dear “muddled” and “inconsistent” spiritual brethren, it is then you discern that what they actually mean by “central and essential” is PERIPHERAL and OPTIONAL. What many heretical boasters do is analogous to an hypothetical Athanasius PROFESSING that the doctrine of the Person of Christ is ESSENTIAL while also considering Arius to be his “inconsistent” spiritual brother. Perhaps a given doctrine of Christ (2 John 9-11) is considered “ESSENTIAL” to these heretical boasters in some wispy, amorphous, or abstract sense.
“No more can it be said that our marks are not proper, but common because heretics (even schismatics) ascribe them to themselves: (1) than if one should say the covenant of God is common to the rescinded and cut off Jews with the Christians because they boast of it; or that the justice of a cause belongs as well to the plaintiff as to the defendant because both claim it. (2) Ad hominem for the same reason, the marks of the Romanists will have to be rejected because not a few besides them ascribe them to themselves (as antiquity, unity, holiness of doctrine and other things of this kind). (3) Nay, no marks of anything in the world can be granted which some impudent and mendacious sophist will not claim for himself. Who is ignorant that the Devil wishes to hold himself as God; that the prince of darkness transforms himself into an angel of light, Antichrist, to arrogate to himself the name of Christ; and the harlot, to conceal herself under the habit of a matron.”