/summa/SS/SSI/SS_Q13:
We must now consider the sin of blasphemy, which is opposed to the confession of faith; and (1) blasphemy in general, (2) that blasphemy which is called the sin against the Holy Ghost.
Under the first head there are four points of inquiry:
(1) Whether blasphemy is opposed to the confession of faith?
(2) Whether blasphemy is always a mortal sin?
(3) Whether blasphemy is the most grievous sin?
(4) Whether blasphemy is in the damned?