Why bother teaching those in denominations?

Answer:

"Son of man, I have appointed you a watchman to the house of Israel; whenever you hear a word from My mouth, warn them from Me. When I say to the wicked, 'You will surely die', and you do not warn him or speak out to warn the wicked from his wicked way that he may live, that wicked man shall die in his iniquity, but his blood I will require at your hand. Yet if you have warned the wicked and he does not turn from his wickedness or from his wicked way, he shall die in his iniquity; but you have delivered yourself. Again, when a righteous man turns away from his righteousness and commits iniquity, and I place an obstacle before him, he will die; since you have not warned him, he shall die in his sin, and his righteous deeds which he has done shall not be remembered; but his blood I will require at your hand. However, if you have warned the righteous man that the righteous should not sin and he does not sin, he shall surely live because he took warning; and you have delivered yourself" (Ezekiel 3:17-21).

Ezekiel was to deliver God's warnings to Israel. He was not sent to the Gentile nations. Israel had God's word. They could read in their Scriptures that they were violating God's commands and that God promised to punish the violators. Notice that Ezekiel was to warn all who were sinning, both the wicked and the righteous.

Look in the New Testament and consider how many teachings and warnings were given to those within the church. It didn't matter that they had access to God's teachings. They needed to be reminded. "Therefore, I will always be ready to remind you of these things, even though you already know them, and have been established in the truth which is present with you. I consider it right, as long as I am in this earthly dwelling, to stir you up by way of reminder, knowing that the laying aside of my earthly dwelling is imminent, as also our Lord Jesus Christ has made clear to me. And I will also be diligent that at any time after my departure you will be able to call these things to mind" (II Peter 1:12-15). Teaching isn't limited to only those who have no idea what the Bible says. It is just as important to remind people who know what God said.

Those in the denominations have spent their lives hearing a distorted message. True, ultimately, they are responsible for their obedience to God, but that doesn't relieve the rest of us from pointing out the truth.

Paul spent some time discussing the lost state of his nation, Israel. They had God's Law, and they were God's people, but they were lost because they rejected the gospel.

"For there is no distinction between Jew and Greek; for the same Lord is Lord of all, abounding in riches for all who call on Him; for "Whowever will call on the name of the Lord will be saved." How then will they call on Him in whom they have not believed? How will they believe in Him whom they have not heard? And how will they hear without a preacher? How will they preach unless they are sent? Just as it is written, "How beautiful are the feet of those who bring good news of good things!" (Romans 10:12-15).

Both the Jews and the Greeks needed to be taught. Access to God's Word did not change the need to teach. But what if they reject the message?

"However, they did not all heed the good news; for Isaiah says, 'Lord, who has believed our report?' So faith comes from hearing, and hearing by the word of Christ. But I say, surely they have never heard, have they? Indeed they have; 'Their voice has gone out into all the Earth, and their words to the ends of the world.' But I say, surely Israel did not know, did they? First Moses says, 'I will make you jealous by that which is not a nation, but a nation without understanding will I anger you.' And Isaiah is very bold and says, 'I was found by those who did not seek me,  I became manifest to those who did not ask for me.' But as for Israel He says, "All the day long I have stretched out my hands to a disobedient and obstinate people'" (Romans 10:16-21).

Everyone deserves the opportunity to hear the truth so that they can make an informed choice. Everyone should be offered multiple chances to hear the truth. This is what God wants, and this is what we do to please our Lord.