The Apocryphon of John, which is part of the Nag Hammadi collection, provides a window into the thought world of Gnosticism, and thereby gives insight into the strange heresy that was beginning to flourish in the Lycus River valley when Paul wrote Colossians. The beliefs Paul refutes in Colossians 2 represent a kind of Proto Gnosticism, which was not as developed as that revealed in the Apocryphon of John, but contains some of the same concepts.
You may read the text of the Apocryphon of John by accessing the following web address: http://www.gnosis.org/naghamm/apocjn.html.
Another fascinating Gnostic document is The Gospel of Thomas, a collection of sayings attributed to Jesus. A few of these are listed below (translated by Stephen Patterson and Marvin Meyer). For the full text, see http://www.gnosis.org/naghamm/gosthom.html. In some you will notice similarities with sayings in the Gospels.
These are the secret sayings that the living Jesus spoke and Didymos Judas Thomas recorded.
1. And he said, "Whoever discovers the interpretation of these sayings will not taste death."
3. Jesus said, "If your leaders say to you,
'Look, the (Father's) kingdom is in the sky,' then the birds of the sky
will precede you. If they say to you, 'It
is in the sea,' then the fish will precede you. Rather, the (Father's)
kingdom is within you and it is outside you.
When you know yourselves, then you will be
known, and you will understand that you are children of the living
Father. But if you do not know yourselves,
then you live in poverty, and you are the poverty."
5. Jesus said, "Know what is in front of your face, and what is hidden from you will be disclosed to you.
For there is nothing hidden that will not be revealed. [And there is nothing buried that will not be raised.]"
7. Jesus said, "Lucky is the lion that the
human will eat, so that the lion becomes human. And foul is the human
that the lion will eat, and the lion still
will become human."
11. Jesus said, "This heaven will pass away, and the one above it will pass away.
The dead are not alive, and the living will
not die. During the days when you ate what is dead, you made it come
alive. When you are in the light, what will
you do? On the day when you were one, you became two. But when
you become two, what will you do?"
14. Jesus said to them, "If you fast, you will
bring sin upon yourselves, and if you pray, you will be condemned,
and if you give to charity, you will harm
your spirits.
When you go into any region and walk about
in the countryside, when people take you in, eat what they serve
you and heal the sick among them.
After all, what goes into your mouth will not
defile you; rather, it's what comes out of your mouth that will defile
you."
22. Jesus saw some babies nursing. He said
to his disciples, "These nursing babies are like those who enter the
(Father's) kingdom."
They said to him, "Then shall we enter the (Father's) kingdom as babies?"
Jesus said to them, "When you make the two
into one, and when you make the inner like the outer and the outer
like the inner, and the upper like the lower,
and when you make male and female into a single one, so that the
male will not be male nor the female be female,
when you make eyes in place of an eye, a hand in place of a
hand, a foot in place of a foot, an image
in place of an image, then you will enter [the kingdom]."
29. Jesus said, "If the flesh came into being
because of spirit, that is a marvel, but if spirit came into being
because of the body, that is a marvel of marvels.
Yet I marvel at how this great wealth has come to dwell in this poverty."
49. Jesus said, "Congratulations to those who
are alone and chosen, for you will find the kingdom. For you have
come from it, and you will return there again."
50. Jesus said, "If they say to you, 'Where
have you come from?' say to them, 'We have come from the light,
from the place where the light came into being
by itself, established [itself], and appeared in their image.'
If they say to you, 'Is it you?' say, 'We are its children, and we are the chosen of the living Father.'
If they ask you, 'What is the evidence of your Father in you?' say to them, 'It is motion and rest.'"
53. His disciples said to him, "Is circumcision useful or not?"
He said to them, "If it were useful, their
father would produce children already circumcised from their mother.
Rather, the true circumcision in spirit has
become profitable in every respect."
56. Jesus said, "Whoever has come to know the
world has discovered a carcass, and whoever has discovered
a carcass, of that person the world is not
worthy."
70. Jesus said, "If you bring forth what is
within you, what you have will save you. If you do not have that within
you, what you do not have within you [will]
kill you."
77. Jesus said, "I am the light that is over all things. I am all: from me all came forth, and to me all attained.
Split a piece of wood; I am there.
Lift up the stone, and you will find me there."
97. Jesus said, "The [Father's] kingdom is
like a woman who was carrying a [jar] full of meal. While she was
walking along [a] distant road, the handle
of the jar broke and the meal spilled behind her [along] the road. She
didn't know it; she hadn't noticed a problem.
When she reached her house, she put the jar down and
discovered that it was empty."
102. Jesus said, "Damn the Pharisees! They
are like a dog sleeping in the cattle manger: the dog neither eats nor
[lets] the cattle eat."
105. Jesus said, "Whoever knows the father and the mother will be called the child of a whore."
106. Jesus said, "When you make the two into
one, you will become children of Adam, and when you say,
'Mountain, move from here!' it will move."
107. Jesus said, "The (Father's) kingdom is
like a shepherd who had a hundred sheep. One of them, the largest,
went astray. He left the ninety-nine and looked
for the one until he found it. After he had toiled, he said to the
sheep, 'I love you more than the ninety-nine.'"
108. Jesus said, "Whoever drinks from my mouth
will become like me; I myself shall become that person, and
the hidden things will be revealed to him."
112. Jesus said, "Damn the flesh that depends on the soul. Damn the soul that depends on the flesh."
113. His disciples said to him, "When will the kingdom come?"
"It will not come by watching for it. It will
not be said, 'Look, here!' or 'Look, there!' Rather, the Father's
kingdom is spread out upon the earth, and
people don't see it."
114. Simon Peter said to them, "Make Mary leave us, for females don't deserve life."
Jesus said, "Look, I will guide her to make
her male, so that she too may become a living spirit resembling you
males. For every female who makes herself
male will enter the kingdom of Heaven."
Selections from Robert J. Miller, ed., The Complete Gospels: Annotated
Scholars Version.
(Polebridge Press, 1992, 1994).
For a bizarre allegorical explanation of the salvation of the soul, you might want to read The Exegesis on the Soul at http://www.gnosis.org/naghamm/exe.html.