"Exhort one another "
"Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin."
(King James Version)
One aspect of fellowship in the light is mutual exhortation. Twice over in the letter to the Hebrews we are told to "exhort one another," with particular reference to the danger of unrecognized backsliding. "Take heed, brethren, lest there be in any of you an evil heart of unbelief in departing from the living God. But exhort one another daily lest any of you be hardened through the deceitfulness of sin." And when we are told not to forsake the assembling of ourselves together, it is again "to exhort one another; and so much the more as ye see the day approaching." We are to help each other to the highest. But we at once recognize the dangers and difficulties of such reciprocal challenging. On the one hand, we could do it in a wrong spirit, as a form of retaliation–to put a person right, to give them what we think they deserve. We can be certain that no one is in a position to challenge another on something in their lives or attitudes who is not equally ready to receive a challenge themselves. Only those who readily and continually "break" and admit their own sins are in a place to point out those of others.
On the other hand, it is truly costly to be faithful to a brother. It is much more comfortable to pass things by, say nothing, and thus not risk disturbing the peace. A challenge may not always be accepted in the spirit in which it is intended. It is costly to give, costly to receive.