"And so the Lord says, 'These people say they are mine. They honor me with their lips, but their hearts are far from me. And their worship of me is nothing but man-made rules learned by rote.'" Isaiah 29:13 NLT
The Lord is talking through Isaiah to Israel and Judah about 600 BC. In this section, the Lord is speaking specifically about Jerusalem and her people.
Jerusalem is the center of the nation of Judah and the location of her temple and the ark (representing the presence of God) where the nation (both Judah and Israel) are to worship and offer sacrifices per the law.
Specifically, the Lord is calling his people out. He says they are "worshiping" him through religious rituals and traditions. And while these things can be helpful, they aren't when they become mindless, external rites and acts. When the people believe that it's the act and not the heart behind it that matters, the Lord objects.
You see, they were saying the right things--doing the outward signs of someone who loved the Lord. But their hearts weren't in it. They were like the husband who brings home flowers for his wife. He stands at the door with a fist full of roses. She answers the door and he says, "Here are roses for you for our 10th anniversary. This is my husbandly thing to do so here you go." I doubt that would illicit a warm response! She wants to hear, "These expensive roses are but a token of my rich and unfailing love for you the love of my life!" And yet, even this act and these words done without a sincere heart is just playacting. The Bible has a word for that--hypocrisy.
So what? Well, I don't think the Lord's words are limited to Israel and the people of Jerusalem. I think we all struggle with our faith becoming a dead religion of pious words and acts. Our heart naturally drifts away from that which is unseen to this world. But the most real things in life are not visible to us. The most beautiful things in creation are often unseen.
So what do we do? We repent. We believe. We turn back to God and admit (if appropriate) that while we are faithful to say the right things and do the right things (sing praise songs at church; read out Bibles) we are not always faithful to focus our thoughts and affections on Him. Then we share our struggles in this area with other brothers and sisters in Christ asking them to pray for us and encourage us and hold us accountable to be sincere and genuine in our worship--both public and private. Perhaps better private worship would lead us to better public worship. After all, who really spends time faking worship when no one is watching? (I know it happens, but it's not near as common)
"Father, I confess my tendency to go through the motions and say the right things. It comes so easily. I confess that I sing familiar songs to you while thinking of other things like what other people think about me or how out of tune things sound or what might be for lunch. I confess that my private worship could be much better. Lord, I repent of such a shallow and dishonoring attitude toward you. I confess my insincerity and hypocrisy. I confess my faith that is more duty than delight. Change my heart! Transform me into someone new! Make me, mold me, fill me, use me for your glory! I love you and there is no other like you! In Jesus' wonderful name I pray, amen!"