How can we tell what belongs to the true faith?
We find the true faith in Sacred Scripture and in the living Tradition of the Church.
The New Testament developed out of the faith of the Church. Scripture and Tradition belong together. Handing on the faith does not occur primarily through documents. In the early Church it was said that Sacred Scripture was “written on the heart of the Church rather than on parchment”. The disciples and the apostles experienced their new life above all through a living fellowship with Jesus. The early Church invited people into this fellowship, which continued in a different way after the Resurrection. The first Christians held fast “to the apostles’ teaching and fellowship, to the breaking of the bread and to the prayers” (Acts 2:42). They were united with one another and yet had room for others. This is part of our faith to this day: Christians invite other individuals to come to know a fellowship with God that has been preserved unaltered since the times of the apostles in the Catholic Church
CCC section (74-87)