A. By the Blessed Trinity I mean one God in three Divine Persons.

Q. 193. {28} Are the three Divine Persons equal in all things?
A. The three Divine Persons are equal in all things.

Q. 194. {29} Are the three Divine Persons one and the same God?
A. The three Divine Persons are one and the same God, having one and the
same Divine nature and substance.

Q. 195. What do we mean by the "nature" and "substance" of a thing?
A. By the "nature" of a thing we mean the combination of all the
qualities that make the thing what it is. By the "substance" of a thing
we mean the part that never changes, and which cannot be changed without
destroying the nature of the thing.

Q. 196. {30} Can we fully understand how the three Divine Persons are
one and the same God?
A. We cannot fully understand how the three Divine Persons are one and
the same God, because this is a mystery.

Q. 197. {31} What is a mystery?
A. A mystery is a truth which we cannot fully understand.

Q. 198. Is every truth which we cannot understand a mystery?
A. Every truth which we cannot understand is not a mystery; but every
revealed truth which no one can understand is a mystery.

Q. 199. Should we believe truths which we cannot understand?
A. We should and often do believe truths which we cannot understand when
we have proof of their existence.