Sabellianism, like Arianism, is a perversion of the doctrine of the Trinity - a very hard doctrine. It is understandable that some confusion might arise, and it is good to have these things spelled out very explicitly.
Modalism exists today. It is said T.D. Jakes is a modalist, and that "United Pentecostals" or "Oneness Pentecostals" all teach modalism.
Anyone who emphasizes "God is one" while saying there are three "modes" or "roles" (rather than the key phrase "persons") is likely a modalist, or at least may be muddled in the direction of modalism. It is the idea that God is one, and that the Father was God in the Old Testament, Jesus was God on earth, and the Holy Spirit is God today (not all three eternally).
Modalism is easily shown to be false:
- Jesus talks to the Father (John 12:28)
- All three persons of God were present at Jesus' baptism (Matt. 3:16-17)
- The witness of heaven is three-fold (1 John 5:7)
Failure to understand the Trinity may lead to worshipping the wrong God (as the Muslims do). It can also damage the marriage relationship (the submission of the woman is related to the submission of Jesus to the Father).