Hebrews 3:13

‘But encourage (‘exhort’) one another day by day, so long as it is called ‘Today’, lest any one of you be hardened by the deceitfulness of sin,’

So important is this that they are to exhort and encourage (parakaleo - comfort, exhort, encourage) each other ‘from day to day as long as time exists’, lest any among them be hardened by the deceitfulness of sin. It is a reminder that while time exists we must ever be on our guard. There can never be a let up in the battle against sin. And we therefore need to ‘encourage and strengthen’ one another (compare Hebrews 10:25).

Note the phrase ‘the deceitfulness of sin’. Compare ‘the deceitfulness of riches’ (Mark 4:19); ‘deceitful desires’ (Ephesians 4:22), things that we crave after which lead us into sin. Sin is seen as something which is out to trap us, like a hunter taking us in his snare, by its enticement and allurements (compare Romans 7:11). It is a reminder that often our sin is not open and deliberate but something that we are lured into because we allow ourselves to be persuaded that it will be good for us or benefit us, being influenced by our strong desire for it. We are led astray. Elsewhere we learn that both our own desires (James 1:14), and men (Ephesians 5:6; 2 Timothy 3:13), and the Devil (Revelation 12:9; Revelation 20:7) deceive us into sin. For sin is the Devil’s ‘power of death’ (Hebrews 2:14)

For we should be aware that, looking at it from the point of view of the long run, sin is never good for us, nor does it benefit us. That is why it is ‘sin’. God does not forbid pleasure, He forbids what will harm us. It may seem to offer present benefit, but we will pay in the end. And once we sin we become more hardened, and if we allow it to go on we become more hardened still. For this is the effect of sin. And sadly for some it may result in simply proving that their faith was not genuine at all, otherwise sin would not have been able to take them over. For those who are His have a Saviour from sin, and sin will not have dominion over them (Romans 6:14).

In the case of the recipients of the letter sin was seeking to deceive them into thinking that they could return to the old ways without it harming them. And they were allowing themselves to be deceived because of peer pressure and fear. So let them beware, for it is the path to disaster!