The training and education of children.

(J. C. Ryle, "The Gospel of Mark" 1857)

"For from within, out of a person's heart, come . . .
  evil thoughts,
  sexual immorality,
  theft,
  murder,
  adultery,
  greed,
  wickedness,
  deceit,
  eagerness for lustful pleasure,
  envy,
  slander,
  pride, and
  foolishness.
All these vile things come from within;
they are what defile you and make you
unacceptable to God."  Mark 7:21-23

The heart is the chief source of defilement and
impurity in God's sight. Our original sinfulness and
natural inclination to evil are seldom sufficiently
considered.

The wickedness of people is often attributed to . . .
  bad examples,
  bad company,
  peculiar temptations, or
  the snares of the devil.

It seems forgotten that everyone carries
within him a fountain of wickedness.

We need no bad company to teach us, and no devil
to tempt us, in order to run into sin. We have within
us the beginning of every sin under heaven.

We ought to remember this in the training and
education of children. In all our management we
must never forget, that the seeds of all mischief
and wickedness are in their hearts.

It is not enough to keep boys and girls at home,
and shut out every outward temptation. They
carry within them a heart ready for any sin,
and until that heart is changed they are not
'safe', whatever we do.

When children do wrong, it is a common practice to
lay all the blame on bad companions. But it is mere
ignorance, blindness, and foolishness to do so. Bad
companions are a great evil no doubt, and an evil to
be avoided as much as possible. But no bad companion
teaches a boy or girl half as much sin as their own
hearts will suggest to them.

The beginning of all wickedness is within!

If parents were half as diligent in praying for their
children's conversion as they are in keeping them
from bad company, their children would turn out
far better than they do.