The NIV is not trying to leave out the fact that Jesus Christ came in the flesh, nor is it a swipe at the divinity of Christ.
1. All of the translations you cited, except KJV, rely on the oldest extant manuscripts available, none of which include the words "that Jesus is come in the flesh." This is just one more reason to take the KJV with a grain of salt, because it does not take into account the oldest manuscript evidence available.
2. 1 John 1:1-4 in all the translations you cited acknowledge that Jesus came in the flesh by stating that they heard him, saw him, touched him. In other words, they sensed his infleshedness with their fleshly senses. The whole point of those opening lines is to attest to his Incarnation/Divinity. So, the added phrase in the KJV doesn't somehow give it a special status in proclaiming the Incarnation that the others don't have. The conclusion of your argument is not only forced, but fallacious.
All that being said, Polycarp does include the phrase "come in the flesh" when he quotes this passage in his letter to the Philippians (as do other church fathers). Polycarp predates any extant manuscript evidence we have. But, before you KJVers get all excited, this doesn't say anything commendable about the superiority of the KJV. It simply means that even a blind squirrel finds an acorn every once in a blue moon.
It certainly and purposely is taking a swipe at the divinity of Christ, as the NIV is based on the corrupted Gnostic Alexandrian manuscripts which Hort and Westcott picked up.
NIV based on Alexandrian Manuscripts - Preserved Word Ministries
Here is a good explanation on the affects of Gnosticism, "...Simon Magus, after his rejection by Peter, began to fashion his own "Christian" church -- a church of which HE was head -- a church designed to completely overthrow the True Church of God. His idea was to blend together Babylonian teaching with some of the teachings of Christ -- especially to take the name of Christ -- and thus create ONE UNIVERSAL CHURCH! But a church with Babylonianism as its basis.
Harnack, a church historian, states that Simon Magus "proclaimed a doctrine in which the Jewish faith was strangely and grotesquely mixed with BABYLONIAN myths, together with some Greek additions. The mysterious worship . . . in consequence of the widened horizon and the deepening religious feeling, finally the wild SYNCRETISM [that is, blending together of religious beliefs], whose aim WAS A UNIVERSAL RELIGION, all contributed to gain adherents for Simon" (Vol. 1, p. 244).
Simon can be classified among the major group of so-called Christians (and Simon called himself such), called by Harnack the: "decidedly anti-Jewish groups . . . . They advanced much further in the criticism of the Old Testament and perceived the impossibility of saving it [that is, the Old Testament] for the Christian UNIVERSAL RELIGION. They rather connected this [universal] religion with the cultus-wisdom of BABYLON and SYRIA" (VoI. 1, p. 246).
With this background, we can understand why Peter so strongly rebuked Simon for his Babylonian ideas. Peter prophesied that this was the man who was to be the "gall of bitterness, and bond of iniquity" to the True Church. Simon’ s attitude was corrupt in the extreme!
The Bible shows he had been working through demons. And yet, he finally called himself a "Christian." Dr. McGiffert, speaking of Simon Magus, says: "His effort to rival and surpass Jesus very likely began after his contact with the Christians that Luke records. His religious system was apparently a SYNCRETISM of Jewish and Oriental elements" (Hasting's Dictionary of the Apostolic Church, Vol. 2, p. 497)....."
While the church at Rome was allowing ancient religious ideas and paganism to creep into its teachings, the church in Alexandria was being corrupted by Greek philosophy and constructing doctrines influenced by Plato and the Stoics:
It is seen in the writings of Clement of Alexandria head of the Catechetical School of Alexandria. He united Greek philosophical traditions with Christian doctrine. He used the term "gnostic" for Christians who had attained the deeper teaching of the Logos which he felt was a lesser form of God, he taught that Christ was not really flesh but spirit. He developed a Christian Platonism, of which objects in the everyday world are imperfect copies. He presented the goal of Christian life as deification, or assimilation into God.
He arose from Alexandria's Catechetical School and was well versed in pagan literature which it seems he used to develop his doctrines. Clement is best remembered as the teacher of Origen who followed him as head of Alexandria's Catechetical School and interpreted scripture allegorically and showed himself to be a Neo-Pythagorean, and Neo-Platonist. Like Plotinus, he wrote that the soul passes through successive stages of incarnation before eventually reaching God. He imagined even demons being reunited with God. For Origen like his teacher Clement, God was the First Principle, and Christ, the Logos, was subordinate to him. He did not believe in the ressurection and taught against that the soul died along with the body, being restored to life only at the resurrection (see soul sleep).
His works were used in the formulation of the early churches doctrines, Origen wrote about 6,000 works. A list was given by Eusebius who studied them and seems to have continued some of the false beliefs which he passed on in his writings. He followed Origen later as bishop of Caesarea and spread his ideas as seen in the further development of the Arian controversies. For instance he was involved in the dispute with Eustathius of Antioch who opposed the growing influence of Origen, including his practice of an allegorical exegesis of scripture. Eustathius perceived in Origen's theology the roots of Arianism and fought against it. He was correct facts were to show, as Eusebius was intent upon emphasizing the difference of the persona of the Trinity and maintaining the subordination of the Son (Logos, or Word) to God. The Son (Jesus), as Arianism asserted, is a creature of God. This Logos, as a derivative creature and not truly God as the Father is truly God, could therefore change (Eusebius, with most early theologians, assumed God was immutable), and he assumed a human body without altering the immutable divine Father. The relation of the Holy Spirit within the Trinity Eusebius explained similarly to that of the Son to the Father. No point of this doctrine is original with Eusebius, all is traceable to his teacher Origen.
So we see where the twisting of the nature Christ begins, and the sources that it came from. It was to confuse and mislead many which we see even to this day......
