If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth:
Have fellowship with him (1 John 1:3) - the essence of the Christian life.
Walk in inward and outward action Walk - in inward and outward action.
In darkness (Greek, 'in the darkness') - opposed to "the light" (cf. 1 John 2:8; 1 John 2:11).
Lie - (1 John 2:4.)
Do not - in practice, whatever we say.
The truth - (Ephesians 4:21; John 3:21.)