That you MUST have? I.e., that you will certainly go to hell without?
Perhaps nothing. That is, it is not a CERTAINTY that you will be damned if you do not become a Catholic. (I am hoping that Jesus might be more expansive than his own words, since HE said that you MUST eat his body and drink his blood in order to have eternal life.)
But even if you might be saved, Jesus wants you to have Himself in the way he commanded, eating and drinking his body and blood, as he says in John, Chapter Six.
Perhaps?
What does the CATECHISM say?
In John, Chapter Six; some fellows asked a VERY direct question.
I would HOPE that Jesus' answer was complete in it's simplicity.
Jesus answered, The work of God is this: to believe in the one he has sent.