"Which belief?"
YaGottaBeKiddinMe!!
THE ONLY belief that REALLY matters: that Jesus IS the Messiah sent from God to personally, with his own body and blood, atone for the sin of all who accept that he acted on their behalf to bear their punishment.
Nothing else that you mentioned has any weight whatsoever in the face of that, for without the Salvation wrought in Christ we all go to Hell; abortionist and pro-lifer alike, whether we use birth control or not, regardless of how many books are in the Bible on our shelf.
You can be a PERFECT Catholic in every aspect of liturgy and practice except actual personal acceptance of Jesus' atoning sacrifice and you WILL go to Hell; all your righteousness counts as nothing.
You can be a PERFECT Protestant in every aspect of theology and practice except actual personal acceptance of Jesus' atoning sacrifice and you WILL go straight to Hell; all your righteousness counts as nothing.
St. Paul himself attested to the overshadowing imporatnce of this, the core belief of Christianity, when he wrote to the Corinthians.
2 For I determined not to know any thing among you, save Jesus Christ, and him crucified.
I Corinthians 2:2 KJV
That is really all that truly matters. A man can be alone on a desert island with no Bible, no church, no Christian bookstore, no seminary, no nothing and if he only makes a personal acceptance of Jesus' atoning sacrifice he WILL go to Heaven.
It really takes all of our earthly bickering about other issues and casts it into a terribly shameful light.