Today we celebrate one of the oldest Christian feasts, the very fact that the the Blessed Virgin's body was taken into heaven to be united with her soul and to reign in heaven with her king, Jesus Christ.
In 1950 this was declared to be a DOGMA, a belief that is required of everyone. If you do not believe it you are outside of Christ's Body, the Catholic Church.
Your can read it here: Pius XII (1950 MUNIFICENTISSIMUS DEUS)
or your can hear it by watching this video: