1 John 1:1-9 - "For Fullness of Joy"

One of the purposes of this Epistle was to stress the full though sinless humanity of Jesus and to assure believers of the certainty of their gift of salvation.  A further purpose was to refute those in the church who were seeking to accommodate Gnostic philosophies and practices into their Christian faith and life. 
You can almost hear John's pastoral heart shining through as he appeals to believers, warning them not to give in to those who teach things contrary to the gospel. Don't deny Jesus. Jesus and the message of the gospel are the only sources of everlasting joy.

As believers we must walk in the light.

There are two distinct groups in Scripture, those who Walk in the Light, and those who Walk in Darkness They are the Saved and the Unsaved. It is the separation of believers from non-believers.  1 Thessalonians 5 succinctly describes this contrast:  Believers Walk in Light, not Darkness; we are Children of the Day, not Night; Believers are Alert, not Asleep; they are Sober, Not Drunk.  To walk in the light, is to follow Jesus.  We are no more children of darkness.  Ephesians 5:8 says, "For ye were sometimes darkness, but now are ye light in the Lord: walk as children of light."  To “walk” is, in short, to live one’s life. One’s lifestyle or way of life can be considered a “walk.” The word also indicates progress. Walking is related to growth; it is taking steps toward maturity. “Light” in the Bible can be a metaphor for life, happiness, righteousness, or understanding. The Bible is clear that light comes from the Lord God, the “Father of the lights” (James 1:17). He is the opposite of evil. Putting it all together, walking in the light means growing in holiness and maturing in the faith as we follow Jesus.