(Heb 3:13) - "But exhort one another every day, as long as it is called 'today,' that none of you may be hardened by the deceitfulness of sin."
In the previous verse, we were challenged to take intentional action, lest the stealthy, relentless assassin of unbelief catch us off guard and slowly lead us away from the Living God. So what is the intentional action that we are to take in prevention against unbelief? The daily exhortation of the word of God from other brothers. God has ordained the daily exhortation from brothers with the word as a means and prevention against the daily assault of unbelief. Satan and our flesh are speaking to us every day, so we must love to receive exhortation from brothers every day.
The clear promise from God here is that by doing so sin will not be able to deceive us into hardening our hearts. This is the case because we have others watching our backs and telling us if they see anything dangerous or needing attention. In this way, godly relationships are exactly the opposite of worldly relationships. In worldly ones (where competition dominates) people are against one another and therefore don't want to be judged or criticized by another.
However, in godly relationships (where the Spirit of love and cooperation dominate) people are for one another's good and therefore invite the protective eye of a fellow brother, knowing his brother many times sees his sin better than he himself does. At the same time, watching the back of another brother is a means to forgetting about ourselves and thereby softening our hearts against the hardening (self-focused) effects of sin.