John Owen assembles many arguments to explain what is going on in Hebrews 6. This one caught my attention:
Fourthly, That it was the manner of the saints, and the apostles themselves, to esteem all baptized, initiated persons, ingrafted into the church, as sanctified persons; so that, speaking of backsliders, he could not make any mention of them any otherwise than they were commonly esteemed to be, and at that time, in the judgment of charity, were to be considered. Whether they were true believers or no, but only temporary, to whom this argument against apostasy is proposed, according to the usual manner of speech used by the Holy Ghost, they could not be otherwise described. (DoD, 255)
Is this satisfactory?