"Christ's ascension" is mentioned in the original Nicene Creed. This text has been important to Christians ever since it was made in 325. It is included in the Mass. It is also mentioned in the Apostles' Creed. It is important for Christian belief because it shows that Jesus' humanity was taken into Heaven. "Ascension Day" is one of the chief feasts of the Christian year. There is plenty of evidence that shows that the feast dates back at least to the later 300s.