Jude 3 urges believers to contend earnestly for what, entrusted once for all to the saints?

Answer: The faith

Jude meant to write about their shared salvation but felt he must urge them to contend for the faith entrusted once for all to the saints.

“Beloved, although I made every effort to write to you about the salvation we share, I felt it necessary to write and urge you to contend earnestly for the faith entrusted once for all to the saints.”— Jude 1:3 (BSB)