I found this good explaination on yahoo answers.
Best Answer - Chosen by Voters
Oh dear, such a lot of anti Catholic sentiment bouncing about on this question and more criticism than plain answers.
You do not HAVE to choose a name to take at a Catholic confirmation, but most people do. The name you take is one of your own choosing and must be the name of one of the many and varied canonised saints of the Catholic Church which includes all the wonderful and great men and women in the Bible!!! The name you choose is based upon a saint who had that same name because they and their life’s journey with God are an inspiration to you; now and hopefully will continue to inspire and encourage you in the years of life ahead.
As for this taking a new name not being scriptural - what a load of nonsense by bigots who clearly do not know their Bible! The model for this re-naming is based on God’s own examples in Scripture when God renamed people when he called them into new life in him, the classic examples are God renaming
Abram to Abraham
Sarai to Sarah
Moses renamed Hoshua to Joshua
Jacob is renamed Israel by God
Gideon is renamed Jerub-Baal by the men of Ophrah
Azariah is renamed Abednego by Nebuchadnezzar’s chief official
Daniel is renamed Belteshazzar by " " "
Hananiah is renamed Shadrach by " " "
Eliakim is renamed Jehoiakim by Pharoah Neco
Lo-Ammi is renamed Ammi, by God
Jesus re named Simon to Peter,
Saul was renamed Paul by God,
Joseph was renamed Barnabas (Acts ch:4).
Renaming at a critical time and point in life and spiritual development / growth is quite a usual occurrence in the Bible and so re-naming at Confirmation is not something that the Catholic Church made up! It is Scriptural and if it’s good enough for God then it’s good enough for the Church!
In all cultures, there are formal and informal rituals to initiate new members, to express who belongs. Christians have the sacraments of initiation (Baptism, Confirmation and Eucharist) to signify and celebrate those who have heard the gospel message and have become disciples – sharing in the mission of the Church, with both the privileges and the responsibilities that come from belonging.
Confirmation is a sacrament of initiation. All Christians believe that each of the rites of initiation is an expression of commitment to the process of becoming adult Christians – or Christian adults. This call for commitment is not something that just happens on the day you are confirmed; it is an ongoing process that continues throughout our lives, but we take the first step on that road at the moment of our confirmation. As with baptism, we become new creations in Christ and this is one of the reason Catholics take a new name, the name of a brother or sister in Christ who has lived before us and who has shown us a way to live in the Christian faith, we take their name to remind us of their journey and hopefully to find their life’s story one to inspire and encourage us to keep going in our own.
If you do not wish to take a name, you do not have to! I know Catholics who did not take a confirmation name. It’s not about the name you take, its about the journey you are undertaking!!!
Besides, God has His own name for you anyhow; the name that no one but God alone knows you by, the name that He has written in the Book of Life, so no matter what name or names you have in this life, God will call you by the name He gave you and you will know it instantly the moment that you hear that name being called!