_I answer that,_ As stated in the Second Part (I-II, Q. 73, AA. 3, 6;
II-II, Q. 73, A. 3), one sin can be said to be graver than another in
two ways: first of all essentially, secondly accidentally.
Essentially, in regard to its species, which is taken from its
object: and so a sin is greater according as that against which it is
committed is greater. And since Christ's Godhead is greater than His
humanity, and His humanity greater than the sacraments of His
humanity, hence it is that those are the gravest sins which are
committed against the Godhead, such as unbelief and blasphemy. The
second degree of gravity is held by those sins which are committed
against His humanity: hence it is written (Matt. 12:32): "Whosoever
shall speak a word against the Son of Man, it shall be forgiven him;
but he that shall speak against the Holy Ghost, it shall not be
forgiven him, neither in this world nor in the world to come." In the
third place come sins committed against the sacraments, which belong
to Christ's humanity; and after these are the other sins committed
against mere creatures.