13:6And he opened his mouth in blasphemes against God, to blaspheme His name and His tabernacle, that is, those who dwell in heaven.

And he opened his MOUTH

This mouth is the one given to him in the previous verse

in BLASPHEMES AGAINST GOD,

This was the same accusation which lead to Christ's crucifixion (Matthew 9:3, Mark 14:53-65).

Jesus indicated that even blasphemy can be forgiven except for blasphemy against the Holy Spirit (Mark 3:28-30). Since the Holy Spirit is part of the triune God, the BLASPHEMY AGAINST GOD seals the "sea beast's" doom.

to BLASPHEME