Another falsehood. If by "RC's" you mean to say Catholics, we already believe in the gospel. Just not in the heretical and myriad self-interpretations so rampant in the cults.
As for joining you (plural), why would we do that, when we have the real deal? The Lord Jesus Christ, Risen Son of the Living Almighty God and Savior of the World, wholly God and wholly man, joins us in the mass every day. Who needs cults/sects/heretics?
What is The Gospel that RC's believe?