It occurs in one other place, (1 Peter 5:2,) and is rendered "willingly;" it is found as an adjective in Philemon 14, and is rendered willingly; and in both instances in opposition to "constraint." So that Schleusner's explanation seems right, "with no compelling force -- nulla vi cogente." It is used in the Sept. for a Hebrew word which means freely, with free will, spontaneously. We may therefore thus render the words, "For if we sin of our own free will, (that is, renounce the faith, which is clearly the sin intended,) after having received the knowledge of the truth, there remains no more a sacrifice for sins."
According to this verse the case of the persecuted is not here contemplated, for they are under constraint; but such are spoken of here as renounced the faith willingly, freely, by their own free choice; so that "willfully" is not what is meant, but spontaneously, without any outward constraining force or influence.
The fathers, such as Chrysostom, Theophylact, and Augustine, sadly blundered on this passage, because they did not understand the sin that is here intended, though it be evidently that of apostasy according to the drift of the whole context; and hence they said some strange things about sin after baptism, though baptism is neither mentioned nor alluded to in the whole passage. How many errors and absurdities have been introduced by the fathers into the world!