This is a warning.  "Watch out!", is to alert the unwary.  It is easy for believers to become complacent as life settles into a routine, when the absence of apparent hazards lulls us into a false sense of security.  The Christian life is not just about wanting to enjoy God's love and benefit from His help now; it is also about serving Him so that He will reward us through eternity.  Hebrews 10:36 says, "You need to persevere so that when you have done the will of God, you will receive what he has promised."

God's reward is principally the joy of being received into His presence, but our minds cannot appreciate the additional blessings He will heap upon us.  Psalm 16:11 puts it like this, "You make known to me the path of life; you will fill me with joy in your presence, with eternal pleasures at your right hand."  We cannot understand now what those delights are but Jesus hints at them in Mark 9:41 and Mark 10:29-31.

Jesus also warned His disciples not to lose their reward by being hypocrites (Matthew 6:1, Matthew 6:5, Matthew 6:16).  Hypocrites are religious play-actors on a spiritual stage trying to impress a human audience … but God is not fooled. The false teachers of John's day pretended to offer the truth, but they did not know it and so served up poisonous lies for believers to swallow.  So the lady to whom this letter is written needed to persevere in the truth and not lose her reward by going off-course.

Navigating by compass across a featureless sea or desert demands precision.  Changing course by just one degree can result in you being a very long way from your intended destination, lost without food or water and about to perish.  John urges us to persevere in the truth so that we will be granted the full reward God has planned for us.  Most of that will not come in this life but will certainly be given to those who love the truth - in the infinite duration of eternity to come (1 Corinthians 3:10-15).