In the Epistle of John, John is referring to the ancient heresies of agnosticism, which claimed that the physical world was inferior and virtually evil compared to the spiritual realm.
Agnosticism is not Christian by origin, its origin is still hidden; however, gnosticism quickly adapted some of the Christian teachings, and rather early even in the first century you had Christian agnostics.
These typically considered the God of the Old Testament to be evil since he created the material realm, while the God of Jesus was considered suppreme since he is saving us from the physical realm and the God of Moses.
Some of these such as Marcion (born 100 AD) even made his own canon of Christian Scriptures (140 AD) which contained most of Luke's Gospels and Paul's Epistles, except for the pastoral.
From a historical point of view this clearly proves external knowledgement of the same Christian canon which we possess today, in year 140.
According to the studies of Agnosticism the reason why Marcion excluded the particular Gospel (Luke) and most of his epistles from the rest of the Canon (New Testament), was due to the internal elements which these writings contained, which agnostics choose to adapt. The matter is however adapted since Agnostics metaphorically interpreted the Scriptures according to their own interests.
Since Paul's first letter to Timothy explicitly refutes and warns against Agnosticism, the letter was left out.
Certain groups within the Agnostic movement were the first to speculate that Jesus Christ did not die on the cross.
They postulated that God took Jesus to heaven and transformed Judas or another individual to resemble Jesus, the poor creature was then crucified and God had deceived the world to believe that Jesus died on the cross.
Most interesting that the same procedure seems to be advocated by the Koran and elaborated on by several secondary traditions and Islamic commentaries.
John deals effectively with influence of Gnosticism upon local Christian churches.
Being an apostle and a prophet, revealing the teaching of God in the same manner of authority as Muhammad (at least to Christians), he brings out a range of doctrinal issues, which are found virtually in most epistles and gospels anyway, but deplorably are seldom referred to in debating Christianity.
The first three chapters of 1 John explicitly explain the nature of a Christian, being spiritually changed and transformed. A Christian lives a holy life, which is the essential evidence to prove ones Christianity.
John also points out that there are many who live a life opposite the Christian faith and standard, which he calls liars, in other words they are not Christians but merely deceive themselves and others.
Even more he states that such have never been Christians in the first place.
This is certainly a lesson for the Muslim, Hindu, Buddhist, Sikh who are unable to understand, that no country is Christian, the WEst is not Christian, being baptised makes no one Christian.
But being transformed to live a Christian life, through the death and atoning work of Christ, by faith and repentance to him resembles a true follower of Jesus Christ.
John even states that such a Christian, will never leave the path and refers to the Christian Gnostics who often engaged in sinful activity since physically related sins did not matter anyway, as having never belonged to the Christian faith.
John further points out that these false Christians could easily be differentiated even though they chose a way of ascethic lifestyle. One needed only to ask them if Christ had appeared in flesh.
To a Agnostic this was outragous since Jesus Christ being God or almost God (depending upon the group or person) could never have been revealed in flesh. Flesh was evil being physical.
Thus according to Gnosticism JEsus was born as spirit and the flesh was merely a spiritual trick, or a spiritual camoflage.
This further explained why some Agnostics rejected the thought of JEsus dying, after all how could a spirit be crucified, and even worse, death, death was the outmost evil in the whole material realm.
The only plausible explanation would be that God took him to heaven and transformed someone else into his likeness.
This however, reveals vital matter, the Agnostics like any other Christian knew perfectly that someone who resembled Christ had died on a cross.
However, this contradicting their own spirit-based concept lead them as the first to reject this historical fact, thus the speculation which is also evident in the Koran.