Wherefore ( διὰ τοῦτο). Referring primarily to Matthew 12:30, and to be joined closely to "I say unto you." Because such is the terrible effect of what you think mere indifferentism, I say this solemnly, Beware of committing the great sin.
Luke's connexion of our verse 43 with verse 30 gives a good but a weaker sense—Become fully decided, lest the devil return to you stronger than ever. Matthew's connexion is—Become fully decided, for the legitimate outcome of want of decision is the sin that will not be forgiven.
I say unto you (Matthew 6:25, note), All manner of; every (Revised Version); πᾶσα. Sin and blasphemy. Genus and species (Meyer). Blasphemy passes in this verse from its wider meaning of open slander and detraction in the first clause to its now commoner but restricted meaning of speech against God in the second clause.
Shall be forgiven unto men: but the blasphemy against the Holy Ghost; the Spirit (Revised Version), thus making it more possible for the English reader to see the connexion of thought with the phrase in verse 28.
Shall not be forgiven unto men. The words, unto men, must be omitted, with the Revised Version. They weaken a statement which in itself may apply to other beings than those that are on earth.