God has shown, in the Bible, His approval with those who follow His will and His displeasure and wrath with those who refuse to worship Him the way He has directed.  God has always been clear about how he is to be worshipped, but man has not listened.  

An example of acceptable and unacceptable worship in the Old Testament is that of Cain and Abel.  Cain and Abel both worshipped God.  Abel did as God directed, but Cain tried to worship God the way he saw fit.  We read in Genesis 4:3-5, “And in the process of time it came to pass that Cain brought an offering of fruit of the ground to the Lord.  Abel also brought of the firstborn of his flock and of their fat.  And the Lord respected Abel and his offering, but He did not respect Cain and his offering.”

Why did the Lord have respect for Abel’s offering and why did He reject Cain’s offering?  Hebrews 11:4 tells us, “By faith Abel offered to God a more excellent sacrifice than Cain.”  Notice Abel offered his sacrifice “by faith” but Cain did not.  Romans 10:17 says, “So then faith comes by hearing, and hearing by the word of God.”  So faith comes by the word of God.  Abel offered his sacrifice by faith or according to the word of God.  If we worship God by faith, we worship as the word of God directs.

Abel offered the firstborn of his flock “by faith” and it was accepted because his worship was according to the instructions of God.  But Cain chose to ignore God’s instructions and sacrificed an offering of the fruit of the ground to the Lord.  Cain chose to sacrifice to God the way he saw fit.  Cain did not offer his sacrifice according to God’s instructions, so God rejected his sacrifice and worship, since it was not “by faith”.

From this example we see our worship must be “by faith”, that is “by the word of God.”  We read in Romans 14:23, “For whatever is not from faith is sin.”  If our worship is not “by faith”, that is it is not “by the word of God”, then it “is sin”.  It will also be rejected just like Cain’s.  It is a very serious matter if God rejects our worship.  If He does we are going to be lost.  Many people accept from the Bible what they like and reject what they don’t like.  They also add other things that they like.  This is not Christianity but religious anarchy.  We do not have the right to “believe as we please.”  We must humbly to submit to all that Jesus says.  Anything taught or practiced by man in the field of religion which does not come from the Bible is false doctrine. 

One cannot obey God without doing what God says to do, when God says to do it, how God says to do it and for the reason God says to do it.  If what we do in worship is not according to the word of God, then it is vain and useless and will cause us to be eternally lost.