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. 
1. Oneness theology rejects the doctrine of the Trinity, for they are unitarian (i.e., believes that God exists as one Person—unipersonal).
2. Oneness theology rejects the eternality of the Person of the Son.
3. Oneness theology rejects that the Son was the actual Creator.
4. Oneness theology rejects the personhood of the Holy Spirit.
5. Oneness theology distorts and thus rejects the biblical concept of the Son being Mediator (Intercessor) between the Father and men (cf. 1 Tim. 2:5). For if Jesus is the Father, then, between whom would He Mediate since by definition a mediator/intercessor represents two distinct partiers, other than Himself. Biblically, only Jesus, God the Son, can rightfully represent the Father (because He is God a distinct Person from the the Father), and represent man because He is fully man. Again, in its proper sense, a "mediator" is one who is other than or distinct from the parties, which are being mediated. However, since in Oneness theology Jesus is both Father and Son, Jesus cannot be properly "Mediator" between two parties--God the Father and man.
6. Many Oneness churches especially the UPCI rejects justification through faith alone by teaching that one must be water baptized (“in the name of Jesus” only) to be saved—with the evidence, as the UPCI teaches, of speaking in other tongues.
7. Virtually all Oneness churches reject that water baptism should be done in the *triune* formal as instructed by Jesus in Matthew 28:19, rather, as they insist, it should be dome in the name of Jesus only.
God: Oneness believers categorically reject the doctrine of the Trinity, thus rejecting the true nature of God—rejecting God Himself. Oneness doctrine holds to the idea that God is a unitarian deity, thus existing as one Person (or unipersonal).
This one unitarian/unipersonal deity (whose name is “Jesus”) has manifested as one of three modes or roles, as either the “Father” (the divine nature of Jesus who is God—eternal), or the Son (the human nature of Jesus, not God—not eternal), or Holy Spirit (synonymous with the Father mode). Hence, Oneness believers firmly deny that there is one God revealed in three distinct Persons. For this reason, believers in this doctrine are called “Oneness” believes. As seen, this belief is also defined as *unitarian* (a one Person God). In an official UPCI doctrinal tract titled, “60 Questions on the Godhead with Bible answers,” we read in question 11: "Does the Bible say that all the Godhead is revealed in one person? Yes, in Jesus Christ. II Corinthians 4:4; Colossians 1:19; 2:9; Hebrews 1:3" (http://www.upci.org/doctrine/60Questions.asp).
Jesus: As indicated, according to Oneness theology, the “name” of this unipersonal deity is “Jesus.” Therefore, as they believe, Jesus, while on earth, had two natures: divine as the Father/Holy Spirit and human as the “Son of God,” though not “God the Son,” only the Father is God. Therefore, Jesus (the name of the unitarian deity) revealed Himself as three different modes or roles.
As a result, Oneness teachers assert that when Scripture speaks of Jesus as God (e.g., John 8:24, 58; 20:28 Titus 2:13; etc.) it is speaking of Jesus in the Father mode. But when Scripture speaks of Jesus as the human man (e.g., “I thirst” or “who touched Me”) it is really speaking of Jesus in the human Son mode. So, when reading Scripture, one must determine, according to this doctrine, in which mode or manifestation Jesus was speaking: the “Father” mode, the human “Son” mode, or the “Holy Spirit” mode?
By asserting that Jesus’ divine nature was merely the
mode of the Father (and Holy Spirit), Oneness believers
are able to proclaim that “Jesus is God in the flesh.”
However, this is a play-on-words, for when they say
“God” what they mean is that Jesus as the Father
is God—namely, His divine nature. In this way, they can
say that Jesus (as the Father) is the eternal God, or
that Jesus (as the Father) preexisted; while claiming
that it was only for the sake of redemption that the
Father (Jesus’ divine nature, not the preexistent Son)
came down and wrapped Himself in human flesh
(though, not actually becoming flesh). In this
view, Oneness doctrine teaches that the Person of the
Son was not eternal His life started in Bethlehem.
For a short outline on the Oneness rejection of the Son's preexistence see this See The Preexistence of the Son and 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 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.