Our Lord says in Mark 7:7, "And in vain do they worship Me, teaching as doctrines the commandments of men." When we follow the commandments of men instead of God’s instructions, our worship becomes vain. If our worship is vain, it is empty, hollow, and useless. It will do us no good. If it is vain, it would be as though we had not worshipped God at all and we will be lost. When we worship in the way "we want" then God is dishonored and insulted. When we worship the way God has told us in His word, God is honored and glorified.
As we read in John 4:24, "God is a Spirit, and they that worship Him must worship Him in spirit and in truth." To worship in truth is to worship as the word of God directs. Jesus said in John 17:17, "Sanctify them by Your truth. Your word is truth." Since the word of God is truth, when we worship God in truth, we worship Him according to His word. The only way God will accept our worship is to follow His directions in the Bible. Any worship, that is not authorized by God's word in the Bible, is not in truth and is vain.
God says in Hosea 4:6, "My people are destroyed for lack of knowledge." If we don’t know what God says, we are going to be destroyed or lost for our lack of knowledge. It is sad to say but the vast majority of people are illiterate of God’s word. They don’t really know what the Bible says. When we don't know God's will then false doctrines will arise which will cause us to be eternally lost.
When people are ignorant of God’s directions on how He is to be worshipped, they wander in spiritual darkness. Our Lord says in Matthew 15:14, "And if the blind lead the blind, both will fall into the ditch." We must first have a desire to know the truth of God's will and then be willing to change when we find we are wrong and are not worshipping according to His truth. Jesus says in John 8:32, "And you shall know the truth and the truth shall make you free." Only the truth can make us free, otherwise we are still under the bondage of sin. We may be sincere in our worship, and we must be, but if we do not worship God "in truth", that is as God directs, then our worship will be vain and unacceptable to God.
Also if we try to worship God "in truth" but not "in spirit", that is not with the proper motive of honoring God and with reverence, then we are just practicing ritualism and our heart is far from it. This type of worship is also vain and unacceptable to God, for Jesus says in Matthew 15:8, "These people draw near to Me with their mouth, and honor Me with their lips, but their heart is far from Me." We must worship God both in spirit and in truth.