Um... so this poem pretty much is straight forward as to the interpretation, however if we look deeper into this poem we can see that "I" represents not just a single person, but all of those whom believes in God and follows God's orders. And if you read carfully enough we can see that this isn't just talking about Christians, it's talking about other religions as well. For example boxes are commonly used in Greek mythology as the jars and sacks are often used in Christianality.
So I'm just basically pointing out that there are lots of interesting ways as to how a poem can be interpreted, just have to look a little deeper and let your mind imagin for you ^^