The YouCat describes a creed as a “bried formula of faith that makes it possible for all believers to make a common profession” (YouCat 26).
The two most used creeds of the Catholic Church are the Nicene Creed and the Apostle’s Creed. In Mass, we pray the Nicene Creed, which was written during two of the great councils of the Church, Nicacea in 325 A.D. and Constantinople in 381 A.D. It’s the Church’s way of briefly summarizing the most important truths of the faith.
But when we talk about the union of God the Father with God the Son, it is not enough to just say that they are the same. They are both God – one God in three unique Persons. By asking us to now use the word consubstantial when we pray the Creed (remember, the Creed is a statement of what we believe as Catholics) the Church is reminding us of the importance of professing that the Father and the Son are the exact same substance.