How does a Christian reject another's theology without rejecting their faith?

I, an "evangelical" "social justice" "protestant" have many theological disagreements with many of my brothers and sisters in Christ. For example, I reject a portion of the Catholic church's teachings. I also can't entirely agree with any church that denies our call to social justice and evangelism. And yet, of course, I admire the faith of many of my catholic or conservative or liberal friends. Many of them know Christ and trust in his death and resurrection for their redemption. I rejoice that I shall spend eternity with them in the presence of the Lord. So my question is this. How do we, all believers, do a better job of being both firm in our theological positions and their importance while also not belittling others' faith? Or even to go further, how do we allow "interdenominational-conversion" to be allowed/encouraged while never maintaining that someone was previously outwith Christ? (Unless they clearly were.)