Ding's comment about the doctrine of the trinity was correct.
The Christian Apologetics & Research Ministry has this to say:
"It is the doctrine that there is only one God, not three, and that the one God exists in three persons: Father, the Son, and the Holy Spirit. The Father isn't the same person as the Son, the Son isn't the same person as the HS, etc."
Modalism would be the nontrinitarian belief that God is a single person who throughout the Bible has revealed himself in 3 modes or forms, first as the Father in the OT, next as the Son at the incarnation, and after Jesus' ascension as the HS.
Info from carm.org