Hearing the truth carries responsibility. Listen to the words of Jesus:
"'If anyone has ears to hear, let him hear.' Then He said to them, 'Take heed what you hear. With the same measure you use, it will be measured to you; and to you who hear, more will be given. For whoever has, to him more will be given; but whoever does not have, even what he has will be taken away from him'" (Mark 4:23-25).
If you learn spiritual truth, and you apply that truth, more truth will be given to you for you to apply to your heart. This is why some people get bored with Bible study, because they never apply the truth to their hearts or their lives. It never penetrates. They just think, "Yes, I kind of understand it. That's sort of an interesting fact."
They never ask God for that truth to transform the way they do business, treat their wives, love their husbands, or speak to the world at large. And eventually, if you're exposed to Bible truth without really making that kind of application, you'll get bored with it. You'll lose what you have. It won't make sense to you. God won't reveal anything to you unless you put into practice what He has already revealed to you. Why should He tell you any more of His secrets?
In Mark 4:26-29, Jesus gives a parable about a man who scatters seed; it grows, but "he himself does not know how" (v. 27). That's how the gospel works. You share the truth of the gospel with somebody, and they listen to it.
You can't see what's going on inside that person's mental processes or inside their heart, but God may be working in their life, and pretty soon there are signs of life. A little leaf starts sticking out of ground, so to speak. You say, "Oh, look; he's interested in spiritual things." Soon, more of the plant comes out, then the full head of grain, and eventually "he puts in the sickle, because the harvest has come" (v. 29). That is, at the right time God reaps that individual into His kingdom. That person eventually comes to know Christ.
A farmer can't make a seed grow. He plants the seed, but he doesn't know what stage of growth it's in until he sees it come up out of the ground. So when we share, we don't know what effect it's going to have, but life may be happening all around us.
You might have some wonderful surprises like I've had with some of the people that I've shared with: They get saved, and you don't know how it happened. Someone may come up to you and tell you, "I got saved because of your testimony to me!" Or maybe you won't know until after you get to heaven. It's okay either way.
But there are two things I want you to take away here: Keep on applying to your life what God tells you when He speaks to you in His Word. Then He will reveal more to you. And keep on sharing with the people in your life, even if they seem hardhearted. You don't know what God is doing inside them.