The question should be: What is the result when a person, (not a Christian), walks in darkness..." - (or continually lives in the lifestyle of sin)...
because those who continually live a lifestyle of sin do not know the true God - until they recognize their sinful life, confess, and honestly ask for forgiveness, and begin to follow Jesus (walk in the light), they are not true Christians at all. These are the exact people John is speaking against in this letter.
1 John 2:3 states, "We know that we have come to know him if we keep his commands. Those who say, 'I know him,' but do not do what he commands are liars, and the truth is not in them."
These hypocrites are lying to themselves and others, and consequently hurting other people immensely as on-lookers conclude Christians are hypocrites or worse - there is no God, or God is evil.
John is not condemning those who slip up every once in a while, as we all unfortunately do, but those with the lifestyle of sin. But what draws the line? We need to continually ask for forgiveness and turn from our sins and not sneak back. Every time. Lay it down at his feet at the cross. Every time.