More seriously, the catechism gives us the basics of Christianity. Being a sinner in need of forgiveness is the universal human condition, not a mark of the Church. By your view, St. Paul destroyed the Gospel over and over again when he said that people who do not provide for their own households have denied the faith or that the congregation should expel the (unrepentant) sexually immoral person from among them, and Jesus did likewise when he said in Matthew 18 to treat the one who refuses to repent of his sin as being outside the Church. Everything except pure universalism denies the Gospel in your view because you think it makes God's grace somehow less than universal and over-arching.
The catechism gives us the basics of living a life as a Christian.
There's a difference between keeping peace and order within a community of believers and being saved by God's grace. The intent in Matthew 18 is always to restore the one into the community. Note also that the issue is not described as "a sin against God," but when a fellow Christians "sins against you." (see also Luke 17:2 about someone sinning against
you seven times in one day!) The way we treat our family, neighbors, and enemies is a witness to the faith God has given us.
I find 1 Timothy 5:8 quite interesting: εἰ δέ τις τῶν ἰδίων καὶ μάλιστα οἰκείων οὐ
προνοεῖ, τὴν πίστιν ἤρνηται καὶ ἔστιν ἀπίστου χείρων. The word translated "provide" (in boldface) is the word, νοέω = "to comprehend something on the basis of careful thought and consideration -
to perceive, to gain insight into, to understand, to comprehend" (Lowe & Nida) + the prefix προ- =
before, in terms of place (e.g., before or in front of other people) or time (e.g., earlier).
It is similar to the origins of the word "provide," which comes from the Latin: pro- = "before" + videre = "to see."
It's about seeing or knowing beforehand what people might need (and then doing something with that knowledge).
What I find quite interesting is that the root of this Greek word: νοέω; is also the root of the word usually translated "to repent," μετα
νοέω. The "change in thinking (and acting)" that is part of being a repentant person; includes "thinking (and acting) beforehand" for the needs of the people around us.
It's a round-about way of saying something I've said for years: the real question is not "What do you believe?," but "What difference does it make in your life that you believe?" How we treat our family members (and I would add, our neighbors and our enemies) is a witness to our faith: the grace that God has shown us in Jesus; and also a witness and an understanding that God's grace in Jesus extends also to all those other people. If that doesn't affect the way we treat others; then it's likely that we don't really believe it.