3 Beloved, while I was very eager to write to you about our common salvation, I was constrained to write to you exhorting you to contend earnestly for the faith which was once for all delivered to the saints.
Jude wanted to write about eternal life in Christ that unites all Christians, but because of false Christians destroying their faith, the message changed.
Contend–Properly, to contend (literally, “struggle upon, appropriately”), i.e. with skill and commitment in opposing whatever is not of faith (God’s persuasion). Strong’s#1864
Jude urges them to fight for the faith and oppose all that which is false doctrine. All Christians are commanded through Scripture to defend the truth.
The word faith in this passage is a reference to the Word of God since it comes from it as seen in Romans 10:17 which says, “17 So faith comes by hearing, and hearing by the word of God.”