By faith Abel offered to God a more excellent sacrifice than Cain, through which he obtained witness that he was righteous, God testifying of his gifts; and through it he being dead still speaks. (Hebrews 11:4, NKJV)
God is not obliged to accept anything and everything people offer to Him in worship. If He is, then God is to be charged with wrongdoing toward Cain. Yet, “by faith” Abel offered “a more excellent sacrifice than Cain”. The Scripture says the Lord “did not respect Cain and his offering” (Gen. 4:5). It is apparent Cain did not bring his offering to God “by faith”. Since faith comes by hearing the word of God, it is evident that Abel followed God’s word concerning the sacrifice; Cain did not (Rom. 10:17). True worshipers are careful to bring before God the worship He approves (Jno. 4:23-24). It is not up to us to decide what is good to give God. We must listen to His word and humbly come before Him with the worship and service He deems acceptable.