Arius was a 4th century priest in Alexandria, Egypt. He denied the deity of the Son of God, holding that Jesus was created by God as the first act of creation and that the nature of Christ was anomoios (“unlike”) that of God the Father. Arianism is the view that Jesus is a finite created being with some divine attributes, but He is not eternal and not divine in and of Himself.
“Arius was a senior presbyter in Alexandria. Around 318 he clashed with Bishop Alexander. Arius claimed that the Father alone was really God; the Son was essentially different from his Father. He did not possess by nature or right any of the divine qualities of immortality, sovereignty, perfect wisdom, goodness and purity. He did not exist before he was begotten by the Father. The Father produced him as a creature. Yet as the creator of the rest of creation, the Son existed ‘apart from time before all things’. Nevertheless, he did not share in the being of God the Father and did not know him perfectly. Arius taught that the Son of God was created ‘out of nothing’… that Christ was of a different substance (heteroousios) than the Father, that is, that He is a creature. Source: What Happened at Nicea?” by James R. White, as published in Christian Research Journal, July-August 1997, Vol. 19 No. 4, pp 30 – 34.
A major misinterpretation in Arianism concerns the meaning of firstborn as applied to Christ. Romans 8:29 speaks of Christ as “the firstborn among many brothers and sisters” (see also Colossians 1:15–20). Arians understand firstborn in these verses to mean that the Son of God was “created” as the first act of creation.
This is not the case. Jesus Himself proclaimed His self-existence and eternality (John 8:58; 10:30). In Bible times, the firstborn son of a family was held in great honor (Genesis 49:3; Exodus 11:5; 34:19; Numbers 3:40; Psalm 89:27; Jeremiah 31:9). It is in this sense that Jesus is God’s “firstborn.” Jesus is the preeminent Person in God’s plan and the Heir of all things (Hebrews 1:2). Jesus is the “Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace” (Isaiah 9:6).
Semi-Arianism admitted that the Son was “of a similar substance” (homoiousious) as the Father, while rejecting that He was “of the same substance” (homoousious). A “similar” essence is still a “different” essence. In the Bible, Jesus is presented as both fully human and fully divine. If He is fully divine, then He is also eternal and cannot be a created being of God the Father. Jesus’ nature is not simply “like” the Father’s; He shares the Father’s exact nature (John 10:30; Colossians 2:9).
Semi-Arianism is not a theologically sound compromise between Arius’s position and orthodoxy. On the issue of Jesus’ divinity, there is no true compromise. Either Jesus was created, or He was not; He is either God in the flesh, or He is not. The Council of Nicaea in AD 325 rejected both Arianism and Semi-Arianism as heresy.
Edit: I realise I have failed to address your question, in what way is Jesus considered to be human according to Arianism. Please allow me to come back with a response as to whether they agree with the Trinitarian concept of the hypostatic union.
The hypostatic union is the term used to describe how God the Son, Jesus Christ, took on a human nature, yet remained fully God at the same time. At the incarnation Jesus, the Word, became a human being, adding human nature to his divine nature:
And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth (John 1:14).
The hypostatic union expresses the orthodox Christian view that at the incarnation Jesus Christ was one Person, fully God and fully man. Jesus is God’s Son in that He was conceived by the Holy Spirit (Luke 1:35). When Jesus was conceived, He became a human being in addition to being God (John 1:1, 14). Jesus is both fully human and fully divine, there is no mixture or dilution of either nature, and He is one united Person, forever.
You ask, was Jesus a spiritual entity cloaked in human biology, or would they (those who follow Arianism) agree with the Trinitarian idea of a hypostatic union? Arius taught that the nature of Christ was unlike that of God the Father, and that Christ was of a different substance than the Father. Semi-Arianism admitted that the Son was “of a similar substance” as the Father, while rejecting that He was “of the same substance”. Either way, neither Arianism nor Semi-Arianism is compatible with the hypostatic union which declares that Jesus shares the Father’s exact nature.
Jesus (the eternal and uncreated Word of God) became flesh to dwell with us. He was God incarnate – one Person, fully God and fully man. Arianism rejects that view.
Second Edit: You now ask: According to Jehovah’s Witnesses in what way is Jesus human? Jehovah’s Witnesses do not deny that Jesus was born as a human and was fully human, up until his death. They acknowledge Jesus as the Son of God, the Christ and the Messiah. However, there are some important differences with regard to who they say Jesus was before he came to earth as a human and who Jesus is now after his resurrection:
Jesus himself said that he lived in heaven before being born as a human... He was God’s first creation... He is the “only-begotten Son.” This means that Jesus is the only one directly created by God. Jesus is also the only one whom God used when He created all other things... [Jehovah] transferred the life of his first-born Son from heaven to the womb of a Jewish virgin named Mary. No human father was involved. Mary therefore gave birth to a perfect son and named him Jesus.
However, on the third day after Jesus died, his heavenly Father resurrected him back to spirit life. Source: Jehovah’s Witness publication ‘What Does the Bible Really Teach’ Chapter 4, paragraphs 10 to 14, pages 41 to 42; paragraph 21 page 46
At times, individuals are known by more than one name... Likewise, the Bible indicates that Michael is another name for Jesus Christ, before and after his life on earth... This scripture (1 Thessalonians 4:16] suggests that Jesus himself is the archangel Michael. Source: Jehovah’s Witness publication ‘What Does the Bible Really Teach’ Appendix pages 218 to 219
Although fully human, Jehovah’s Witnesses believe the pre-mortal Jesus was created by Jehovah God and is not equal to God. His body was not resurrected and what came out of the tomb was a spirit creature who took on the appearance of a man before returning to heaven as a spirit.
To answer the second part of your edited question: Would Jehovah’s Witnesses agree with the Trinitarian idea of a hypostatic union?
Trinitarian Christians understand that Jesus always had been God (John 8:58, 10:30), but at the incarnation Jesus became a human being (John 1:14). The addition of the human nature to the divine nature is Jesus, the God-man. This is the hypostatic union, Jesus Christ, one Person, fully God and fully man. The Trinitarian view of the hypostatic union declares that Jesus shares the Father’s exact nature. The divine nature was united with the human nature in the person of Jesus Christ.
In light of the above, it is clear that Jehovah’s Witnesses must disagree with the hypostatic union.