Theological Identification: “Oneness” theology, historically known as “Modalism.”
Who: Oneness churches include the United Pentecostal Church International (hereafter UPCI), which is the largest Oneness domination. In addition to the UPCI, there are many other Oneness churches having generic names such as “Apostolic,” “Bethel Temple,” “Higher Ground,” “Jesus’ Name,” or even “Jesus Only,” etc. Further, there are many popular and prolific preachers on the airwaves that propagate Oneness theology (e.g., Trinity Broadcasting Network [TBN] features one of the most recognized Oneness preachers, T. D. Jakes of the Potters House, Dallas, TX. 
The basics of Oneness theology:
The Father: Jesus’ divine nature, God.
Son: Jesus’ human nature, the Son of God, not God the Son, for only the Father is God. Jesus’ divine nature is the Father (or the Holy Spirit), His human nature is the Son.
Holy Spirit: Jesus’ divine nature. Thus, as to His divine nature, Jesus is both Father and Holy Spirit depending on His particular function (e.g., Jesus as the Father created all things, but Jesus as the Holy Spirit mode is the Comforter).
Oneness Christology is a clear and major departure from biblical orthodoxy. It removes the personhood and deity and incarnation from the Son, thus removing the Son from the Trinity. The chief Oneness Christological divergences from that of the biblical teachings are as follows:
§ Oneness Christology denies the unipersonality of the Son, Jesus Christ
§ Oneness Christology denies that the “Son” is God. As stated, Oneness theology teaches that Jesus’ divine nature represents the Father and Holy Spirit, but not the Son—i.e., the “Son” is not God; He is merely the human nature of Jesus (see Bernard, 1983: 99, 103, 252; UPCI, 2008b).
§ Oneness Christology denies the preexistence and incarnation of the Son and His role as the agent of creation (cf. Bernard, 1983: 103-4; Magee, 1988: 25). Thus, by denying the preexistence of the Son, Oneness doctrine rejects the incarnation of the divine Son holding to the erroneous notion that it was Jesus as the Father, not the Son, who came down and wrapped Himself in flesh (hence, not actually becoming flesh), and that flesh was called “Son” (see Bernard, 1983: 106, 122; 1991:103).
§ Oneness Christology claims that Jesus is the Father, Son and Holy Spirit (same Person), hence denying the concept of the Trinity (cf. Bernard, 1983: 57; Weisser, 1983: 2; UPCI, 2008b). Thus, Oneness theology is “unitarian” seeing God as a unipersonal deity, same as Muslims and Jehovah’s Witnesses.
In sharp contrast, in the Pauline corpus, the deity, unipersonality and distinction of all three Persons are seen frequently either in the same passage or same context (cf. 2 Cor. 13:14). Both John and Paul present the Son as fully God and Creator of all things (cf. John 1:1, 3, 18; Col. 1:16-17; Phil. 2:6-11).
In the prologue of Hebrews, the author in v. 3 and God the Father Himself declares that the Person of the Son is "the God" (ho theos, v. 8) and further, God the Father *directly addresses* the Son as "Lord" (using the vocative kurie, “Lord”), thus, the YHWH of Psalm 102:25-27—the unchangeable Creator (1:10-12). The Father’s words here totally demolish the Oneness position, which sees the “Son” representing only the human nature of Jesus whose life started in Bethlehem.
Oneness theology cannot stand exegetically. It must circumvent and redefine the plain reading of many passages that state or imply, for example, the grammatical and contextual distinctions of the Persons in the Trinity, the preexistence and deity of the Son, and the deity and unipersonality of the Holy Spirit. Further, contrary to the historical revisionism frequently employed by Oneness authors and teachers, the early church prior to Nicea held to the concept of the Trinity and universally rejected Oneness doctrine, that is, they rejected both modalistic and dynamic forms of Monarchianism.
We must pray that God rescues Oneness believers from the darkness of Oneness-unitarianism. As Christians, we must present to them a coherent presentation of the gospel proclaiming the one true God and Savior of biblical revelation, who has revealed Himself as triune.
For a short outline on the Oneness rejection of the Son's preexistence see this See The Preexistence of the Son and Oneness Theology.
 Oneness theology was first known as monarchianism, which comes from the Greek word monarchia, meaning single principle. There were two forms of monarchianism: modalistic, and the far less accepted, dynamic (or more properly called adoptionism), both of which emerged at the end of the second century. Modalistic monarchianism, known also as Modalism, Sabellianism (named after the heretic Sabellius, who came to Rome and taught it at the beginning of the third century) and even patripassianism (from Lat., meaning, “father to suffer”). Today, however, Modalism is generally classified as “Oneness.” Modalism earned its name from its distinctive theology. Basically, Modalism (or Oneness theology) teaches that God is a unitarian (i.e., unipersonal), indivisible monad. Hence, the titles “Father,” “Son,” and “Holy Spirit” were merely the different modes, roles, or offices that the unipersonal deity temporally manifested for the sake of redemption. Oneness teachers today tell us that Jesus is the name of the single, lone Person behind the three masks of the “Father,” “Son” and “Holy Spirit” (in contrast to early Modalism, which taught it was the Father Person behind the masks).
 If you are unsure about the orthodoxy of a particular church (or pastor), examine the church’s doctrinal statement concerning God. If it avoids the word “Person,” and/or describes God as three “manifestations” or “dimensions” (as T. D. Jakes does, see The Potter's House use extreme caution! Orthodox Christianity has never described God as merely temporary appearances, manifestations, or even worse, “dimensions.” Oneness churches typically describe God in those terms. However, if a church claims to be Trinitarian, yet uses terms like “manifestations” to describe the three Persons of the Trinity, it reveals theological ignorance or carelessness. In my observation, the term “manifestations” in a doctrinal statement frequently indicates Oneness rather than Trinitarian theology. Therefore, when churches avoid the term “Persons” in their doctrinal statements—beware.