Romans 10:17 (NKJV)
What would you do if you bought a tube of toothpaste, brought it home, and the when you squeezed it ketchup came out? That would be really strange and no one would like that. When you buy a tube of toothpaste you expect to get toothpaste, but why is that? Why is there toothpaste in the toothpaste tube and not something else? It is not a trick question and the answer is really very simple. There is toothpaste in the toothpaste tube because that is what was put in there.
In Romans 10:17 we learn how we get faith, through hearing God's Word. As we continue to spend time hearing God's Word, through teaching or preaching and our own reading or studying it, we are building our faith. But what people often don't realize is that while this principle works in the area of building our faith, it also applies to all of the things we hear and spend time with. Just like spending time in God's Word will build our faith, what we spend our time with will build something in us.
People become afraid because they hear things that build fear into them. People become depressed because they hear things that make them sad. In the New Testament we are repeatedly encouraged to hear the right things and avoid the wrong things because of what each will do to us and for us. To build faith, we need God's Word, and if something we hear does not agree with what God's Word has told us, then that thing will not build faith in us and we should avoid it. What we spend our time hearing, and even thinking about, will do something in us.