John Lightfoot on Luke 12:9

[But he that denieth me, &c.] consider whether in these words and in the following verse, our blessed Saviour do not point at those two unpardonable sins, apostasy, or denying and renouncing of Christ, and blasphemy, or the sin against the Holy Ghost. The first is called "a sin unto death." And so, in truth and in the event, is the latter too. I find them, indeed, confounded by some, who discourse upon the sin against the Holy Ghost, when yet this difference may be observed, viz., that apostasy cannot properly be charged on any but who have already professed Christianity: but blasphemy against the Holy Ghost was uttered by the scribes and Pharisees at that time that they disowned and rejected Christ.