And their message will spread like cancer, Hymenaeus and Philetus are of this sort,
who have strayed concerning the truth, saying that the resurrection is already past; and they overthrow the faith of some."
2 Timothy 2:16-18 (NKJV)
If you wanted to have good, healthy teeth, but you never brushed your teeth, and ate a lot of snacks that were filled with sugar, should you be surprised when the dentist tells you that you have several cavities? If you go outside without an umbrella while it is raining and you get wet, should you be shocked that this happened to you? If you are, then you have not recognized that you have done something that led you to less than desirable results, and you need to make a change in what you are doing.
In the two letters that Paul wrote to Timothy, Paul talked about things that Timothy should and should not do. Something that Paul mentions more than once is how the wrong kind of teaching and ideas need to be avoided, because of the negative results that these things can bring. In 2 Timothy 2:16-18, Paul talks about the wrong kind of teaching and ideas as something that will increase ungodliness, overthrow people's faith, and Paul even compares this kind of thing to cancer!
That is very strong language and leaves no room for doubt about what happens when people spend their time listening to the wrong things. Good, Bible based teaching will increase godliness, build people's faith, and rather than being harmful like cancer, right teaching builds us up and equips us to live the life that God has planned for us. We need to be sure we are spending time in God's Word and hearing good teaching. If we are not, we will have some not so shocking, negative results.