we all talk about 'believers' and how their walk should be. What is a 'believer'? What is it a believer believes in? Here is where the line gets drawn. A non-catholic does not believe the things a catholic believes. And vice-versa. These are not small things, either. Salvation, sanctification, eternal security, The Gospel of the Grace of God. On the other side, we have works for salvation, the power of priests to remit/retain sins, the keys to the kingdom, traditions, eucharist, Mary, pergutory, prayers for the dead, the Pope, etc.
Obviously, we have completely separate BELIEFS. We are NOT BELIEVERS of the same message. These are so opposed that they cannot find a common ground in false humility and prayers for what brings us together. Nothing brings us together. The name of Jesus? Paul talks about "another Jesus" being preached.
Not only that, we are not called to yoke ourselves to unbelievers. It would be a farce to pretend otherwise. We are told to be ready to preach the Gospel. In season and out. And stand in truth, not sit in quiet disagreement.
Ah, yes ... but we have the unfortunate tendency to forget that we are merely messengers, and not the Message itself.
And where God's truth is concerned the messengers can be, and very often are, quite offensive. To the point where our bad behavior blinds people to "THE TRUTH" we purport to be telling them.
As St. Francis put it, "Preach the Gospel always; when necessary, use words." How we behave, matters.