Lines 13-16 Summary

Lines 13-14

   That if I chance to hold my peace,
   These stones to praise thee may not cease.

• We're back to iambic tetrameter here (which means these lines contain four iambs).
• The speaker tells us essentially that he's written this poem just in case something happens to him. If he chances to hold his "peace," the "stones" that make up the poem will continue to praise the Lord long afterwards.
• These lines allude to a famous passage in the Gospel of Luke, where Jesus says that, even if his disciples were to stop praising the Lord, the stones themselves would cry out. 
• Now this whole little bit about "peace" is interesting here. On the one hand, it means, essentially, to stop talking, as it does in the passage from Luke (and as it does in wedding ceremonies).
• It can also have a more sinister meaning, though, and refer to death. If the speaker dies, or loses his voice, then the poem will act as his surrogate and praise God even if he's "quiet."
• Okay, we have to ask, if these lines are perhaps also about death, what's with all this "chance" business? Isn't death the only guaranteed thing in life?
• Well yes—come to think of it, death is the only guaranteed thing in life. So what's the deal?
• We're gonna go ahead and say that the speaker is thinking a little about his eternal life and all that here.
• If the speaker is lucky and makes it to heaven and is able to live forever, then he will never technically "hold" his "peace." 
• If he doesn't make it however, then, yes, he will not have eternal life and will definitely be holding his peace (keeping quiet).
• So, the poem is in a way the speaker's second self. It is an altar and offering, one that praises God and will continue to praise God, just like the speaker has presumably done throughout his life, for years to come. 
• Now we're pretty sure you've already figured this out, but "stones" continues with the whole stone-altar image that's been floating around throughout the poem.
• We also know that the speaker has compared his heart to a "stone" too, so once again we are reminded that the altar refers to both the poem and something more metaphorical: the stones that comprise it are the pieces of the speaker's heart. 
• His heart is his offering, and it also infuses the poem itself. He's put his whole heart into it, you could say (and, hey, we just did).

Lines 15-16

Oh, let thy blessed SACRIFICE be mine,
And sanctify this ALTAR to be thine. 

• At last we've come to the end of the "The Altar," and to the end of the altar (get it?).
• Once again, the speaker addresses God directly and asks for God's sacrifice to be his (the speaker's), and for God to bless and sanctify his altar.
• Woah okay, now hold on a second. Just what sacrifice did God make, if we may ask?
• Ah, this is a reference to Jesus Christ, the Son of God, who died upon the cross in order to redeem mankind.
• God knew this was going to happen, and while he wasn't exactly okay with it, he knew that it was necessary (and that Jesus was going to come back to life three days later anyway).
• The point is, this was a sacrifice on God's part.
• So then does the speaker want to sacrifice his own son? Eh, not exactly. What he really means is he wants to make a similarly difficult sacrifice.
• In other words, he's saying something like this: "God, let me take up my own cross, my own burden, and carry it and suffer for your sake."
• Sure, he doesn't say that outright, but usually when a super-Christian poet talks about sacrifice like this, he's getting at this idea. (Read more about it right here, if you like.)
• Now we know the speaker has been talking about his altar-building project as a sacrifice this whole time, and we also know that is a difficult and painful project (tears, anyone?).
• Building an altar out of his own heart and then offering it to God is the speaker's way of taking up his own cross.
• It is his way of sacrificing his time and enduring hard labor in order to praise God and give him everything he has. Since he's put all this time and effort into the project, he wants God to accept it and bless it ("sanctify") as a worthy sacrifice.
• He wants God to say, essentially, "Good job sir, I accept this sacrifice, and give it my full support, errr, blessing."
• We've come full circle at this point. Not only does the speaker go back to the iambic pentameter with which he began, he's back to talking about sacrifices (remember all that business about tears?).
• He's made an altar, only this altar is no ordinary altar. It's the human body itself—the speaker's heart, the center of his emotional being.
• He's arranged it into an altar, and thus turned it into something that he can offer to God, even if it is broke, hard, imperfect, and all that.
• This offering isn't just a heart, but a poem in praise of God as well. 
• The altar is both the speaker and his work of art. You could say it's everything he has and everything he does.