In the first reading St. John warns us against those who do not acknowledge Jesus Christ as coming in the flesh.
In the time of the Apostles there were many people who felt that Jesus was too fine to have had flesh like the rest of us. That mistake was rooted in an old Persian belief in two creators. There was supposedly one who created everything good and spiritual, and one who created everything evil and fleshy. Those people, rather than believe there was anything evil about Jesus, began saying that what appeared to be his physical body was actually a mirage. Taking the Greek word for a mirage, which was dokein, such people were called the Docetists.
Even though they had good intentions in saying there was nothing fleshy about Jesus, their mistake actually detracted from his greatness. Jesus took on a real body because he had to be one of us in order to redeem our human nature. What is more, by saying that his body was not real the Docetists were denying the great heroism Our Lord experienced in willingly suffering the horrors of his humiliating execution.
By taking on our full human nature he elevated the dignity of all of us.