[stonehouse01]

As Belloc writes in The Great Heresies - there is no essential doctrine such that if we can agree upon it we can agree to differ about the rest; as for instance, to accept immortality but deny the Trinity.

Meaning - Truth is not relative, it is whole - it cannot be picked apart - and it is found in the Catholic Church.

[Steelfish]

Exactly. As BXVI pointed out in his address to World Youth Day:

“Truth is not to be found in a slogan, an idea, or an ideology. It is found in a person, in the person of Jesus Christ.”

From there on we must accept the rest of His teachings including the establishment of His one Catholic Church to reveal His Truth. Anyone propagates differently, puts forward a heresy. And Belloc goes on to explain with skillful brilliance the how, what, and why, other religions are heresies pure and simple.