Some forms and modes of worship are not acceptable to Almighty God. The reason Cain's offering was rejected while Abel's was accepted stemmed from the condition of the heart (presumptuous and self-willed as opposed to repentant and contrite). God will not accept our worship of him if our heart is not right- not yielded (or grounded) in spirit and truth, totally guileless and pure. Our worship of God will only be accepted if our hearts are totally yielded to His spirit doing His will with no hidden agendas or ulterior motives.