A whole litany of 'I believe' (credo in Latin) is a fixed part of the Roman Catholic mass. The users profess their belief in God, some of the dogmas of the Christian faith, the Holy Church, etcetera.
-- from I Believe by rp
I believe are the first two words to apostle's creed , a formula containing in brief statements, or articles, the fundamental tenets of Christian belief, and having for its authors, according to tradition, the Twelve Apostles. The present day creed thought to be composed between them with each of the Apostles contributing one of the twelve articles, was commonly thought of in the Middles Ages as created on the on the day of Pentecost, while still under the direct inspiration of the Holy Spirit. Although no one article can be specifically assigned to the authorship of a separate Apostle, it was believed that it was the joint work of all, implying that the deliberation took place on the day of Pentecost. Later religious leaders speculated, "they for many just reasons decided that this rule of faith should be called the Symbol", which is a Greek word to mean both indicium, i.e. a token or password by which Christians might recognize each other
More recently, some have assigned to the Creed an origin much later than the Apostolic Age, that in its present form it represents only the baptismal confession of the Church of Southern Gaul, dating at earliest from the second half of the fifth century. Strictly speaking, the terms of this statement are accurate enough; though it seems probable that it was not in Gaul, but in Rome, that the Creed really assumed its final shape. The Apostle's Creed is the most commonly used rite in Western Christianity for daily prayer and worship,used in some worship services to describe the faith into which one is baptized.
Some churches give the option of using either the Apostles' Creed or the Nicene Creed, suggesting the Nicene Creed as the more festive or solemn of the two, using the Apostle's Creed in standard worship services and the Nicene Creed in worship services where the Sacrament of Holy Communion is observed. The Nicene Creed was written by the early Church and adopted (in a slightly different version) by the Church Council at Nicæa in AD 325 then further revised to its present form by the Council at Chalcedon in AD 451.
As a reference see:The Small Catechism of Martin Luther where Martin Luther (1483-1546) included a brief explanation of this Creed in his Small Catechism, Part 2
Sources:
Catholic Encyclopedia
ORIGIN OF THE CREED:
http://www.newadvent.org/cathen/01629a.htm
The Nicene Creed :
http://www.ortech-engr.com/churches/ch_general/nicene_creed.html
The Apostles' Creed:
http://www.ortech-engr.com/churches/ch_general/apostles_creed.html