The Bible tells us Cain’s offering from the ground was not accepted by God, but his brother Abel’s, from the flock, was (Gen. 4:3-5). Why was one accepted and the other rejected? Both made an offering, to the same God, for the same purpose. Cain obviously wanted to please God or else he would not have been upset about his offering being rejected.
The New Testament tells us specifically why Cain’s offering was not accepted—it was not offered by faith (Heb. 11:4). To do something by faith is to do it according to God’s word (Rom. 10:17). Thus, Abel did what God said to do, Cain did not. Cain did what he wanted to do and expected God to like it.
Many people today do exactly what Cain did back then—they hear part of what God says and then adapt it to their own liking and expect God to accept it. One example of this is in musical worship. The New Testament says to sing; speak, teach, admonish (Eph. 5:19; Col. 3:16). However, they hear God wants musical praise and thanksgiving, and alter it by adding instrumental music. They offer this to God and think He ought to be satisfied with it. After all, they say, “It sounds so good. It really stirs me up and draws me closer to God.” The only evidence of this is their testimony. It may sound good to the ear, but God looks on the heart. It may stir emotions within man, but God is not man—His ways are not our ways, nor are His thoughts our thoughts (Isa. 55:8, 9).
Cain was condemned because he offered worship to God his way, not God’s. Any and all who strive to worship God their own way stand condemned as well. It does not matter how sincere their motives, nor how moving the experience—it is sinful and not accepted by God.