What is the theme of La Celestina?

There are many! For me, personally, these are the big ones:

-The hopelessness and impossibility of love between people of different religions (the tragic nature of courtly love)

-The priceless value of female honor, a.k.a. virginity

-Greed as both a motivating factor and a cause of downfall

-The view of love as a "sickness" with debilitating effects both physically & mentally

-Misogyny or the popular medieval view of women as corrupt seductresses

-The fear and hysteria caused by the Spanish Inquisition

-Putting individual differences aside in pursuit of the "greater good"

-The powerful influence of others' thoughts -- reputation -- on your own actions (as opposed to one's own morals and beliefs)

-Societal changes in accordance with the transition into the Renaissance (individualism, business, the end of chivalry, graphic violence, torture as a means to a political end)

-The wisdom associated with old age

-Doing what one must just in order to survive (prostitution, servitude, deception, etc.)

-The old concept of "cleanliness of blood," a.k.a. lacking Muslim heritage