"Repent" is the first word that proceeded out of Christ´s mouth when He began His ministry, and it is the first thing that He tells us to do; because without repentance is no remission of sins.
To repent is to change our mind, and change our heart, and change our will. When Christ told the adultress woman, "Go and sin no more", this is what repentance is; and we first must have a willing mind, because sin begins in our thoughts, which if we do not stop, ends in our actions.
If a mans sin is looking on a woman to lust after her, then let him make a covenant with his eyes, so that he will no longer think of it. If our eye is single, as of one; an eye focused on the things that belong to God, then we are full of light; but if our eye is insincere toward God, then we are full of darkness. We must rather do as Christ tells us, and go and sin no more; no longer willfully, deliberately sinning. God looks for sincerity, and knows those who are, and are not sincere; He know either by their sincere effort or their insincere effort.
The sincere person may fall several times, but will always rise back up into their sincerity toward God; but it is the wicked, or the insincere, who fall into mischief.