I know that most Christians will say that they are sinners, but so many will point to someone who commits a “big” sin (mortal sin) and will say that that person is obviously not a Christian.
Where does this idea come from? Since almost everyone I’ve seen make this claim is a non-Catholic Christian, they must be getting this idea from the Bible (they don’t accept Tradition as authoritative) Where in the Bible does it say that Christians don’t commit “big” sins?
I think the problem here is where do we identify “big” and “small” sins?
The beloved disciple John often talks on the subject of those who claim to be Christian and yet do not do Christian things:
He who says, “I know Him,” and does not keep His commandments, is a liar, and the truth is not in him. [1 John 2:4]
He who says he is in the light, and hates his brother, is in darkness until now. [1 John 2:9]
Do not love the world or the things in the world. If anyone loves the world, the love of the Father is not in him. For all that is in the world—the lust of the flesh, the lust of the eyes, and the pride of life—is not of the Father but is of the world. [1 John 2:15-16]
So when we see, say, Benny Hinn lying on television about raising a man from the dead, backtracking later on about it, etc, we can point and say, “That is not a Christian,” if he chooses to continue and embrace it. At the same time, we cannot be too judgmental, for there really is no such thing as “big” or “small” sins.
For whoever shall keep the whole law, and yet stumble in one point, he is guilty of all. [James 2:10]
Furthermore, we will be judged the way that we judge others: “For with the same measure that you use, it will be measured back to you” (Luke 6:38). This doesn’t mean we can’t denounce wrongdoing (the apostles and Church Fathers all did this), but that we must be discerning of how we go about it. Our Lord instructed that if we see a brother sinning, for example, we should confront him privately about it (Matt 18:15), and Apollos was confronted privately by Aquila and Priscilla regarding some errors he had in his teachings, and from that he grew into a spiritually sound individual (Acts 18:24-28).
So yes, when I see someone professing to be Christian committing bad deeds, I do say, “That is not a Christian,” or perhaps more appropriately, “That is not a Christian thing to do,” because I am simply stating it for what it is. At the same time I pray for that person that they may see the light and return to the true Way of Christ, so that we may gain a brother.