“Eternal” means “lasting or existing forever; without end or beginning” — it’s a classic adjective for declarations of love and fidelity. But you could make your oath stand out with the synonym “sempiternal,” meaning “eternal and unchanging; everlasting.” The Latin roots of this word are powerful; “semper” means “always,” and it’s joined with “aeternus,” or “eternal,” to represent an enduring, everlasting presence.