Keats was always thinking about life, death, and his place in the pantheon of great poets. Since "Ode on a Grecian Urn," is partly a meditation on the timelessness of art, the poem parallels Keats' own suppositions about the timelessness of his own poetry and ideas of immortality in art and the afterlife.
Borrowing Keats' concept of "negative capability," which requires the poet/reader to be more open to interpretations, I hesitate to categorize the poem in terms of "sacred" and "profane." These carry too much religious or ethical baggage, although I certainly do see the connection these terms can make. Since immortality and timelessness are two pressing themes in the poem, I tend to think of the sacred as immortal, spiritual or ideal and I think of profane as common, earthly or sensual. Thus, the spiritual (sacred) implies the eternal (permanant) while earthly and sensual experiences imply transience.
Keats' poetry and the urn are artifacts. What is eternal? What is earthly? The urn is ancient. Over time, Keats' poetry, if it is preserved, will last a long time; perhaps forever. These artifacts are earthly but have a timelessness to them. So, what makes these artifacts paradoxical: permanent and transient?
If anything is a bit profane in the poem, it is the image of the lovers about to consummate their love. The opening line, "Thou still unravish'd bride of quiteness" illustrates that the bride has not yet been deflowered. Notice the word "still" which could mean "not yet" and/or "motionless." She is always already about to make love but never will. She is frozen in anticipation. In a sense, her purity (or sanctity to use your term "sacred") is immortalized by the fact that she is frozen in time, never to actually make love. That earthly delight of sex itself (which never occurs), if it were to occur would be fleeting, certainly fleeting relative to immortality. More than any other image, this shows the preservation of the sacred and the transience of the profane.
In a larger sense, Keats shows that the images in the urn only symbolize timelessness. The figures on the urn are immortalized in their actions, but those actions are frozen. The speaker comes to this conclusion of the frozen, yet lifelike scene in the 5th stanza, calling the images a "Cold Pastoral!" They are, in a sense, dead in their immortality. Real immortality is beyond the physicality of humans or their artifacts. Art is the attempt to tune into abstracts like the eternal or truth.
The hotly debated line "Beauty is truth, truth beauty," in the 5th stanza, can be interpreted to be an aphorism for this view of art; that art can be timeless but this can only give you a glimpse ("tease") of actual immortality. Truth here is eternal and beauty is more sensual and earthly. However, this line can be interpreted in many other ways.
If "still," in the first line does mean "not yet," then the earthly pleasure of love might be satisfied in the abstract or in an afterlife. A lover of poetry and having died at 26, art and immortality were two of his main concerns: that is, whether earthly truths provide glimpses of absolute (permanent) truths.