For I bear them record that they have a zeal of God, but not according to knowledge.
For they being ignorant of God's righteousness, and going about to establish their own righteousness, have not submitted themselves unto the righteousness of God
(Romans 10: 2-3, KJV)
"Follow your heart". "Do what makes you happy". "Treat them how they treat you".
These are all sayings based in people establishing their own righteousness. We aren't to follow our own heart because Jeremiah 17:9 tells us that the heart is deceitful above all things. Because of this, we are to trust in God and lean not unto our own understanding per Proverbs 3:5-6.
We aren't to do what makes us happy. We are to deny ourselves and take up our cross daily and follow Christ per Luke 9:23. Doing what makes us happy may involve engaging in sin, which is of course something we should avoid. We aren't to treat people how they treat us. We are to bless our enemies. Per Luke 6:32-34, if we only love those who love us, then we are no different than worldly people.
Yet, the same people who tell us to "follow our hearts", "do what makes us happy" , and "treat others how they treat us" may very well have a zeal for God. They may reference God and speak highly of Him. They may love God and desire a relationship with Him. Yet, they are ignorant of God according to knowledge if they stand by the above statements.
We cannot just have a zeal for God and expect that to be enough. Zeal with no knowledge is catastrophic. When we love God but don't take the time to attain knowledge of God, we not only lead ourselves astray, but we can also inadvertently lead others astray by saying things that run contrary to the Word and Will of God. God tells us to study the Word of God so that we can rightly divide the word of truth.
Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth.
(2 Timothy 2:15, KJV)
It is impossible for us to speak truth if we don't read the truth. Yet, it is possible to have a zeal for God and speak things that are not based in the righteousness of God. Proverbs 14:12, states there is a way which seems right unto a man, but the end thereof are the ways to death. People, please be careful about doing and saying things that seem right to you... because it could very well be wrong.
I do encourage you to have a zeal for God. But a zeal for God with no knowledge is just like eating empty calories, it has no benefits and serves no purpose. Recall, Jesus told Satan that man should not live by bread alone, but by every word that proceeded out of the mouth of God (Luke 4:4). In short, if you aren't reading the word of truth, you have no knowledge of God. Without eating your daily bread, you are running on fumes and zeal, which won't get you very far.
So, fill up on knowledge to accompany any zeal that you may have for God. Zeal is good! Excitement for God is beautiful because it acknowledges the deity and greatness of God. But to adequately acknowledge God, we must have knowledge of God. Fumes and zeal won't cut it. To make it in this life, you will need something that will last. Zeal, life, heaven, and earth will all pass away, but God's Word will always stand (Matthew 24:35). Until we couple our zeal with ever-lasting truth, we will continue to run on fumes and zeal.