Catholics and Reformed Protestants believe something very similar regarding original sin. Bryan Cross has an excellent series on Aquinas and Trent that does a good job of explaining the Catholic doctrine on this subject and a few others.
So if Mr. Cross "does a good job of explaining the Catholic doctrine on this subject" what need is there of the Catechism of the Catholic Church? The APOSTOLIC LETTER LAETAMUR MAGNOPERE states:
Catechesis will find in this genuine, systematic presentation of the faith and of Catholic doctrine a totally reliable way to present, with renewed fervor, each and every part of the Christian message to the people of our time. This text will provide every catechist with sound help for communicating the one, perennial deposit of faith within the local Church, while seeking, with the help of the Holy Spirit, to link the wondrous unity of the Christian mystery with the varied needs and conditions of those to whom this message is addressed. All catechetical activity will be able to experience a new, widespread impetus among the People of God, if it can properly use and appreciate this post-conciliar Catechism.I guess the CCC left out some things, so Mr. Cross can help out.