Something sacred is set apart for some religious purpose. Secular is the opposite of sacred but a true antonym for this word depends on the context.
The goddess Artemis' sacred tree is the cypress tree.
Value of sacred heart of Jesus
The sacred python was the idea of the sacred untouchable python in Things Fall Apart, expressed by the Igbo tribe. The python was considered an animal of the gods and was not to be harmed or interfered with.
'Opposite' has no comparative or superlative form. Something is either opposite something else (or opposite to something else), or it is not.
The opposite is exterior (outside).