Longfellow sonnet on Miguel de Molinos

THREE Silences there are: the first of speech,

The second of desire, the third of thought;

This is the lore a Spanish monk, distraught

With dreams and visions, was the first to teach.

These Silences, commingling each with each,

Made up the perfect Silence that he sought

And prayed for, and wherein at times he caught

Mysterious sounds from realms beyond our reach.

O thou, whose daily life anticipates

The life to come, and in whose thought and word

The spiritual world preponderates,

Hermit of Amesbury! thou too hast heard

Voices and melodies from beyond the gates,

And speakest only when thy soul is stirred!

UPDATE:  As Vance points out, the poem is dedicated to Whittier.  Here is the Molinos passage in question:  

Tres maneras hay de silencio: el primero es de palabras, el segundo de deseos y el tercero de pensamientos. El primero es perfecto, más perfecto es el segundo y perfectísimo el tercero. En el primero, de palabras, se alcanza la virtud; en el segundo, de deseos, se consigue la quietud; en el tercero, de pensamientos, el interior recogimiento. No hablando, no deseando ni pensando, se llega al verdadero y perfecto silencio místico, en el cual habla Dios con el alma, se comunica y la enseña en su más íntimo fondo la más perfecta y alta sabiduría.

In other words: first simply stop speaking, that is the perfect silence leading to virtue. The more perfect silence is to quiet desires, producing quietude. (Remember Molinos is known for "quietism.") The third, the even more perfect silence, leads to withdrawal to one's interior.  All three together produce perfect mysticism where God speaks directly to your soul with the highest and most perfect wisdom. 

I don't know much about Whittier aside from his two famous poems, but he also did some translations from the Spanish.  

UPDATE 2:

We could see Longfellow as a translation of a sort from the Molinos.  It is a paraphrase / adaptation at least, and involves the transfer from one language to the next. It is in the tradition of the poetic gloss.