As a Catholic you believe that the Catholic Church is the one, true Church established by Jesus Christ. Do not waste your time trying to show how good or holy or intelligent Catholics are. Stick with the fact that that God's grace is far more powerful than man's sin and that Christ is faithful and loving to his bride, the Church.
Jesus Christ intended to establish a particular Church, not a loose affiliation of like-minded believers. His intention was that his Church would go out into the world as a united body, exercising his own three-fold ministry as prophet, priest, and king.
The Church is his body, having authority and being able to guide (Lk 10:16; John 16:12). The Church has the authority of Christ himself (John 13:20; 14:16-17). The Church, the People of God, is made up of both good and bad members (Mt 13:24-30) will exist until the end of the world (Mt 28:20).
Read Mt 16:18-19 and 18:15-18. It is clear that the Church has an authority that is supersedes that of the individual.
The Church is a sign of unity. Read John 17:20-21. Who can doubt Christ's great desire that he expressed the night before he died for us? Who can doubt the scandal we give to non-Christians when they look at us, not only strong and ancient Churches, but thousands and thousands of insignificant groups of a few members each, who all claim to be the only ones to follow Christ truly? If you are a Catholic, be proud that you belong to the Church that has, for the last two thousand years, taught the fullness of the Christian faith.
It is not enough - because it is not the mind of Christ as expressed in the Bible - to just believe in Jesus Christ as Lord. The Bible gives us other ideals:
1. "I appeal to you, brethren, to take note of those who create dissensions and difficulties, in opposition to the doctrine which you have been taught; avoid them. For such persons do not serve our Lord Christ, but their own appetites, and by fair and flattering words they deceive the hearts of the simple-minded." (Romans 16:17-18)
2. "I appeal to you, brothers and sister, by the name of our Lord Jesus Christ, that all of you agree and that there be no dissensions among you, but that you be united in the same mind and the same judgment." (1 Corinthians 1:10)
Unity is not just a unity of nice feelings. It is a unity of teaching what has been handed on to us since the time of Christ.