The following is an excerpt from
The MacArthur New Testament Commentary on Hebrews 3.
But encourage one another day after day, as long as it is still called “Today,” lest any one of you be hardened by the deceitfulness of sin. (3:13)
Encourage is from the Greek parakaleo, a form of the word used by Jesus of the Holy Spirit in John 14:16. The root meaning has to do with coming alongside to give help. The writer is saying to the believers among those to whom he is writing, “Get along side each other and help each other.” They are especially urged to help their unbelieving Jewish brethren by encouraging them not to harden their hearts but to accept Jesus as the Messiah.
Deceitfulness means “trickery” or “strategem.” Sin is tricky; it seldom appears as it really is. It always masks itself. It lies and deceives (cf. Rom. 7:11). When a person becomes spiritually hardened, he rarely is aware of it. He can hear the gospel of Jesus Christ time and time again and not respond. My father often used the well-known expression, “The same sun that melts the wax hardens the clay.” If your heart is not melted in faith, it will be hardened in unbelief.
The old nature constantly suggests that sin is not as bad and that trust in Christ is not as important as the Bible says. Becoming a Christian seems too costly, too demanding, too restrictive, too drab and unexciting—and, above all, unnecessary. From one’s own perspective, he does not seem so wicked. “I take care of my family, I am a helpful neighbor and a good citizen. I’m not perfect, of course, but I’m not evil, either. My life has room for improvement, but it doesn’t need ‘saving.’ ” So the thinking goes. This is what the sin nature deceitfully tells men about their need for salvation.
God’s assessment is quite different. “But My righteous one shall live by faith; and if he shrinks back, My soul has no pleasure in him. But we are not of those who shrink back to destruction, but of those who have faith to the preserving of the soul” (Heb. 10:38–39). There we have it. You stand on the edge of decision, a decision which you cannot escape. Either you believe to the saving of your soul or you fall back to damnation.