It seems John Howard is
determined to put his Methodist upbringing firmly behind him. Why else
would he congratulate the new Pope for taking on the responsibility of
leading the world’s “Catholics?” The Prime Minister’s words are no
doubt well meant but they’re also ignorant. The new Pope does not head
the world’s Catholics. Like his predecessor, he is the leader only of
the world’s Roman Catholics. There are many millions of Catholics who
are not Roman Catholics and who do not recognise the authority of the
Pope.

Fowlers Modern English Usage (2nd edition) makes the point clearly:

It is open to Roman Catholics to use Catholic by itself in
a sense that excludes all but themselves; but it is not open to others
to use it instead of Roman Catholic without implying that no other
Church has a right to the name of Catholic. Neither the desire for
brevity (as in the Catholic countries) nor the instinct of courtesy (as
I am not forgetting that you are a Catholic) should induce anyone who
is not Roman Catholic to omit the Roman. The words should not be
hyphened.

John Howard should not use the term Catholic when he means Roman Catholic.