Herbert is often called a "devotional poet," which means his poetry is characterized by, well, devotion to God. If you take a quick gander at some of his poems right here, heck even just the titles, you'll see what we mean. "The Altar" is no exception to this general rule; it has all the hallmark sounds of devotional poetry.
Okay, so what does that mean? Well, did you notice that the poem kind of sounds like a prayer, or an offering? Check out these lines to see what we mean: "Oh, let thy blessed SACRIFCE be mind / And sanctify this ALTAR to be thine" (15-16). That whole bit about sanctifying the "ALTAR"? Yeah, that's just like saying "accept this sacrifice God, and bless it as my humble offering." The same goes for the lines where the speaker says, "That if I chance to hold my peace / These stones to praise thee may not cease" (13-14). That definitely sounds like this famous children's bedtime prayer. Let's not forget the poem's opening lines: "A broken ALTAR, Lord, thy servant rears, / Made of a heart and cemented with tears" (1-2). Those lines are right up there with any number of different prayers and blessings that have a little "Lord" or "Oh Lord" right at the beginning, like this one that you may have heard.
All the long-vowel sounds in this poem definitely add to its devotional flavor. For example there's the lone O sound in "alone" and "stone," then there's the long E of "cease" and "peace," the long I of "mine" and "thine," and of course the long A of "frame," "same," and "name." Long vowels are a little more serious than short vowels. They take longer to pronounce (hence why they are called "long"), and there's something more reverent or solemn about them (try saying the word "alone" a few times aloud to yourself to see what we mean). They are the perfect addition to a poem that is about building an altar and offering oneself to God, a solemn and serious topic if there ever was one.