O friends! remember this once for all

(Thomas Brooks, "A Cabinet of Choice Jewels" 1669)

"Cleanse me from my hidden faults." Psalm 19:12

"I hate vain thoughts." Psalm 119:113

A gracious soul conflicts most with heart-sins, and is
most affected with spiritual sins, and laments and
mourns most over secret sins—invisible sins—sins
which lie most hidden and remote from the eyes of
the world. He is most affected and afflicted by inward
pollutions and defilements.

Grace will rise and conflict against the most inward
and secret vanities of the soul, such as—
  secret self-love;
  secret hardness of heart;
  secret unbelief;
  secret carnal confidence;
  secret hypocrisy;
  secret envy;
  secret malice;
  secret vain-glory;
  secret fretting and murmuring;
  secret lustings;
  secret runnings-out of the soul after worldly vanities;
  and secret pride.

True grace makes opposition as well against the
being of sin in a man's nature—as against the
breakings out of sin in a man's life!

True grace will make war against the corruptions of the
heart—as well as against the excursions of the feet!

True grace is as willing and desirous to be rid of a
polluted heart—as it is willing and desirous to be rid
of a polluted hand.

True grace would gladly have, not only sinful acts—but
also sinful dispositions; and not only irregular actions—
but also inordinate affections—mortified and subdued.

O friends! heart sins are root sins! Certainly a proud heart
has more of Satan in it than a proud look! And a lustful
heart is more vile than a lustful eye! Therefore true grace
makes war against heart sins, against spiritual sins, against
the most inward secret sins—against those very sins which
do not lie within the reach of the piercing eye of the most
knowing or observing man in the world—but are only
obvious to an Omniscient eye!

Spiritual convictions can reach to the most inward, secret,
spiritual, and undiscernible sins. Certainly that is a sincere
heart, a heart more worth than gold—which smites a man:
  for inward sins—as well as for outward sins;
  for sins done in secret—as well as for sins done in public;
  for spiritual sins—as well as for fleshly sins;
  for sins against the soul—as well as for sins against the body.

O friends! remember this once for all—that the main
battle, the main warfare of a Christian lies not in the open
field, it lies not in visible skirmishes. But his main quarrels
and conflicts are most within, and his worst and greatest
enemies are those of his own house—those of his own heart.
A little grace may reform an evil life—but it must be a great
deal of grace that reforms an evil heart! A little grace may
make a man victorious over outward gross sins—but it must
be a great deal of grace that makes a man victorious over
inward sins, secret sins, spiritual sins, heart sins!

"Search me, O God, and know my heart; test me
 and know my thoughts. Point out anything in me
 that offends You, and lead me along the path of
 everlasting life." Psalm 139:23-24