‘And have no fellowship with the unfruitful works of darkness, but rather even reprove (convict) them, for the things which are done by them in secret it is a shame even to speak of.'
Living in the light means avoiding the darkness and its ‘unfruits'. They must not have any part with the behaviour of those who are still in darkness. But it is their behaviour they must avoid, not them (this means non-Christians, not defaulting Christians). Indeed by the purity of their lives they should act as a reproof and a way of convicting such people about their manner of life so that they may be converted. That this means primarily by life and not by word comes out in the last phrase. Christians should not even talk about, or think about, such behaviour as they witness in others. They must refute them by their lives. Thus men will see their good works and glorify their Father who is in Heaven (Matthew 5:16).
‘The unfruitful works of darkness.' In direct contrast with ‘the fruit of the light' (Matthew 5:9). That which is done in spiritual darkness can be of no lasting benefit.