Simple and straightforward:
* The universe holds many eternal mysteries.
* The love between a mother and child is eternal.
Adding depth with context:
* The ancient temple stood as a testament to the eternal power of faith.
* Her spirit seemed eternally young, even though she was well into her 80s.
* The artist's vision sought to capture the eternal beauty of the natural world.
Figurative language:
* The poet's words echoed with an eternal resonance.
* His love for her burned with an eternal flame.
* The inscription on the tombstone spoke of eternal rest.
Note: The word "eternal" implies something that lasts forever, without end. This can be a literal concept (like the universe) or a figurative one (like a love that never fades). Choose the usage that best fits your intended meaning.