I will keep it simple for you.
Jesus left Peter to start his church and shepherd his flock. This Church and it's people took on the names of Christians and Catholics from the 1st century.
Jesus' entrusted the teachings of the people to his Church with the help of the Holy Spirit. Jesus' church to fight many falsehoods taught in the 16th century by Luther and Calvin and Zwingli. The Church that Jesus is building put together a book to help the many people being led astray from the teachings of Jesus and this book is called the Catechism.
So the Catechism is simple to read guide for the secular world to help them to know what Jesus' church (the Catholic Church) teaches so they can better know him and how to live. These teachings come from the Apostolic Traditions both WRITTEN and ORAL. The Written Traditions were accumulated over a couple of centuries and decided on which were canon (inspired by God) by the Catholic Church. The Catholic Church eventually decided on 27 New Testament books and the Septuagint for the Old Testament and from the late 4th century until the early 5th century put together the Vulgate.
As to Judgment day... that is for Jesus to decide. Some are going to Heaven and the rest get to spend eternity in Hell.