There are many lessons that we can learn from Cain and Abel(Genesis 4:1-12). Cain was jealous and angry with Abel, because it had gone so well with Abel (for God had accepted him).
How do you feel when you find it going very well with someone whom you are closely associated with? When you find a fellow-brother who is preaching victory over sin and you find that his life is consistent with his preaching - that there is no evidence of anxiety or anger or impatience in him, but on the contrary, a perpetual spirit of triumph and joy and peace; and in contrast your own life is miserable and gloomy and defeated (because you do not believe that a life of victory is possible) - Do you then find a root of envy creeping in and a desire to see him fall somewhere so that you can gloat over his failure and drag him down in the eyes of others? That was the same spirit that drove Cain. Do you feel happy when some trouble befalls that brother? Does his perpetual victory irritate you and make you angry? That attitude of yours, is the spirit of Cain, not the Spirit of Christ; and the sooner you recognise it for the foul, slimy thing that it is, the better.
Sin was crouching at the door of Cain's heart when God sought to warn him. Sin wasn't a hundred miles away but at the door of his heart pressing for entrance. Cain was NOT thinking about doing something good. He had not yet thought to do something evil, for in that case, sin would have been inside the door already. He had just kept his heart empty, swept and unoccupied with anything good. That opened the way, as Jesus said for eight wicked spirits to occupy it (Matt.12:43-45).
There was no joy in Cain's life, for such a man cannot truly rejoice. His face was fallen, and gloom and depression and anger were written across it. He could not bear to see it going so well with someone else. Cain was angry with God and with Abel. An extra reason for his anger was that it was going well with someone younger than him. It would perhaps have been tolerable to see it going well with someone older, but it was unbearable to see it happening to someone younger.
Before you throw a stone at Cain, consider: Can you truly rejoice when you see brothers who are much younger than you, going ahead of you spiritually? Do you seek their good so zealously that you are eager to see them go ahead of you. If so, then you have the mind of Christ Who could rejoice that His disciples would do greater works then He Himself did (Jn.14:12). But "where jealousy and selfish ambition exist, there is disorder and every evil thing" (Jas 3:16). Yes, where the spirit of Cain is found, every imaginable evil will also be found, sooner or later.
When Cain invited Abel for a walk in the fields (Gen.4:8), he went along with him unsuspectingly. Abel did not suspect that Cain was jealous of him - or else he would not have gone out with him. How simple in heart Abel was. He had a broken and contrite heart and did not judge his brother Cain. What an example for us to follow!
How often even those with whom it is going well in other ways, are suspicious that others are jealous of them or that they are thinking bad thoughts about them or trying to push them down etc., We may have nothing against others, but may yet suspect that others are having things against us. Thus we also are defiled. Abel did not even suspect one who was out to murder him. What a good heart he had! That is the type of heart we need. There are two cliffs on either side of the narrow way. One is the cliff of jealousy and the other is the cliff of suspicion - and we can fall over either cliff and reach the bottom. Abel was neither jealous of Cain nor suspicious of him. This is the path we are to walk in relation to our fellow-believers. It is better to die like Abel with an unsuspecting heart than to live with a heart contaminated with suspicion. The one who suspects and judges others of being jealous or angry, is a busybody in the matters of others, and thereby proves that he himself does not have a broken and contrite heart any more then the one who is jealous and angry.
If we have lowly thoughts about ourselves, like Abel had, then we will not even think that others could be jealous of us. We would then be so insignificant in our own estimation that we would not imagine that anyone could possibly be jealous of us.
Abel rejoiced in God's acceptance of him. But that did not lift his heart up in pride against Cain. No. He would not compare himself with Cain neither would he be a busybody in Cain's life.