This is one of the great mystical poems in the language. A soul — speaking as a woman in love — leaves her house in the dead of night, in the dark and in disguise, with no guide but the light burning in her breast, to meet the Beloved in secret. It is the union of the soul with God told as an elopement of love.
San Juan wrote these lines, and a long prose «declaration» explaining them, after a real night: the months he spent imprisoned for reforming his order. The poem turns that darkness into the very road to the light — «oh night that guided!» — in eight liras of a sweetness never equalled since.
The verse is kept in Spanish. Translating it in passing would replace the work with a paraphrase of it.