Modalism or Sabellianism is a heresy that began in the 3rd century by a theologian named Sabellias who rejected the Orthodox Trinitarian doctrine of the Trinity that states that the Trinity consists of three distinct persons namely, Father Son and the Holy Spirit, both simultaneously co-equal ,co-existing and one in essence.
Modalism view of the trinity is that the persons of the GodHead are not distinct in number but in functions or modes, they claim that God manifests in three persons but yet being One God more like putting on a mask. God is said to appear sometimes as Father, sometime as Son and also as The Holy Spirit but not simultaneously. The Father is said to be the Son and the Son the Holy Spirit and so on. This view however claims to teach monotheism but at the same time denies the Trinity which is a biblical doctrine, modalists view the protestant Trinitarian view as that of tritheism.
Modalism differ from Unitarianism in that modalism affirms the Deity of Christ while the Unitarianism rejects it. They base their beliefs on Colosians 1:15 20
Col 1:15 Who is the image of the invisible God, the firstborn of every creature:
Col 1:16 For by him were all things created, that are in heaven, and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities, or powers: all things were created by him, and for him:
Col 1:17 And he is before all things, and by him all things consist.
Col 1:18 And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.
Col 1:19 For it pleased the Father that in him should all fulness dwell;
Col 1:20 And, having made peace through the blood of his cross, by him to reconcile all things unto himself; by him, I say, whether they be things in earth, or things in heaven.
And John 14:9
Joh14:9 Jesussaith unto him, Have I been so long time with you, and yet hast thou not known me, Philip? he that hath seen me hath seen the Father; and how sayest thouthen,Shew us the Father?
Turtullian called this view Patripassianism, Since there is no distinctions between the persons in the GodHead, this implies that The Father also suffered on the cross and The Holy Spirit suffered too, the father and the Holy Ghost also incarnated during the Son incarnation. Someone also has to answer the question that to whom did Jesus pray to every time He prayed? Whose voice sounded during Jesus baptism?
The Church has always rejected this heresy and still rejecting it today although many confessing Christians still hold to this unbiblical view.
Soli Deo Gloria