In this poem, the speaker describes what the process and practice of writing poetry—of being a poet whose heart is like an "open house" to all the world—is like.
How the Vulnerability of Poetry Affects the Poet
In the first stanza, the speaker describes how he really can have no secrets because they "cry aloud" for everyone to hear. We might assume, through his writing, that this is the reason he has "no need for tongue." He does not speak his secrets; rather, they come out in his poems. His heart, he feels, is like an open house, with doors wide open for any and all comers to enter. There is a vulnerability here, certainly. He has no disguises: he cannot. Thus, we arrive at the idea that to be a poet is to be constantly vulnerable, to have no secrets from the world.
The Importance of Spiritual Vulnerability
In the second stanza, the speaker says that all his truths are "foreknown," meaning that these truths already exist, but he writes poetry in order to reveal them more clearly to the rest of us. The "anguish" of this difficult labor reveals itself in the work. The poet feels "naked to the bone," another expression of his vulnerability and openness to the world. However, at the same time, this nakedness is his "shield" and must offer him some level of protection; he "wear[s]" himself, he says, keeping "the spirit spare." Perhaps he must work to open his "self" in order to keep his spirit open to receiving and transmitting these truths. So it is that, to write poetry, one must be spiritually open, vulnerable to truths that might cause pain.
The Pain of Spiritual Vulnerability
In the third and final stanza, the speaker seems to describe a lack of control: he doesn't claim the "anger" he feels or "deed" he does as his own, and so they seem to come from outside and within him simultaneously. He can stop liars with his "language strict and pure" (i.e., the truths he reveals). We see, again, that the sacrifices necessary in order to be a poet are painful, the process of writing is laborious and agonizing at times.