The speaker addresses his "poor soul" and calls it the center of his sinful earth (a.k.a. his sinful body). That's right, Shmoopers, this dude is talking smack... to his soul. He thinks his soul is being a total chump these days. Instead of being the one who's in charge of telling the body what to do, it's been letting his body give all the orders. He wants to know why his soul is suffering inside while it spends so much time and effort worrying about outward looks and materialistic stuff.
He reminds his soul that life is really short and that his body is totally going to die someday. Not only that, but worms are going to devour his corpse and all the other material possessions that the body leaves behind. (Like, say, all those fancy duds and pieces of jewelry people like to be buried in?) Gross.
The speaker wants his soul to turn things around ASAP. He advises it to let his body suffer while it works on developing a rich, inner spiritual life. He also tells his soul to spend its time and effort making sure it will live for eternity instead of spending all its time doing a bunch of scummy stuff on Earth. (Yeah, that's a little vague. No word from the Speaker on how his soul should go about doing that exactly. Should he pray more? Do a bunch of good deeds?)
Finally, the Speaker says that if his soul does what he says, it will totally be able to defeat death. He says that, even though death "feeds" on men's bodies, his immortal soul will never die. Bonus.