The book of Hebrews, though commonly called an eptistle, is more like a sermon. It's full of exhortations, some extremely difficult/controversial passages, some key powerful verses, and one particular message - Jesus is greater. He is greater than angels, Moses, the priesthood, the sacrificial system, etc. Though its author remains unknown and its original audience uncertain, its message is powerful. To a people finding it difficult to hang on to the Christian faith because of opposition and persecution and considering returning to their Jewish religion, the author encourages them to not give up, because Jesus is greater than anything they are tempted to go back to.
The book of Hebrews also has an encouraging and correcting message for the church today. Amidst all the distractions, discouragement, and weariness, it challenges us to hold on to Jesus because he is greater!