Ave Maria!
Sabellianism taught that the 3 Persons in the Trinity were not distinct persons but rather 3 different modes or offices that the 1 God appeared in. Knowledge of their error is helpful in dealing with those who believe that the Bible is the sole transmitter of the Deposit of Faith. Sabellius broke from Sacred Tradition and it was this that orthodox opponents used to combat him.
Ave Maria!
To Download Audio go to http://airmaria.com?p=10470