You cannot use other people's instrumentals with your music lyrics and, say, record it and post it on a website or attempt to publish it in any other way. If you, say, attempt to recite a poem in your home with other people's instrumentals with no attempts to make a recording and publish it, you'll be okay. On the other hand, older works that have been around for many, many years become public property at some point, so, therefore, the same rules do not apply. I would look for instrumentals that have been around for a very long time, such as Bach or Mozart, and I'm pretty sure there are some Christian tunes and old hymnals that are not copyrighted. It's just a matter of a simple internet search to find which ones you can use and which you cannot.