To "contend" for the faith means to fight on behalf of the faith.
Those Jude was addressing were already of the saved, so his intention was not to motivate them to acquire faith but to fight for it.
[Jde 1:1 KJV] 1 Jude, the servant of Jesus Christ, and brother of James, to them that are sanctified by God the Father, and preserved in Jesus Christ, [and] called:
The point was that Christ was completely faithful and obedient to the Father, and Rev. 2:13, regardless of what you may think it means, demonstrates that faith existed within Him - His faith. You just shouldn't ignore nor misappropriate a verse because it demonstrates something you don't like or don't agree with. Look at Gal 2:16. Here again we are informed of Christ's faith.
[Gal 2:16 KJV] 16 Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified.
[Heb 12:2 KJV]
2 Looking unto Jesus the author and finisher of [our] faith; who for the joy that was set before him endured the cross, despising the shame, and is set down at the right hand of the throne of God.
How is it that you think a verse of scripture is only good for "opinion"?
[2Ti 3:16 KJV] 16 All scripture [is] given by inspiration of God, and [is] profitable for doctrine, for reproof, for correction, for instruction in righteousness:
Yes, of course, we need to see it by the context to understand what the Bible says. Christ speaks of the doctrine like that of Jude “which was once delivered unto the saints” for he speaks about the matter of the teachings or beliefs of Balaam or the Nicolitans. He was not speaking of his personal faith.
Now, this is how the phrase “faith of “was used in early Modern English. The KJb Translators did not use as you think to be, it’s far from the truth. They were speaking of something not related to the personal faith of Christ but something that signifies the
teaching/ doctrine of the Church or the belief of the church. The “of” while it is true may refer to belongingness but can also be rendered as “pertaining to” as per Oxford Dictionary. Below are two examples used by the KJB Translators pertaining to the subject “faith of”.
§9 The translating of the Scripture into the vulgar tongues
• 1 Now though the Church were thus furnished with
Greek and
Latin translations, even before the
faith of CHRIST was generally embraced in the Empire: [S.Hieronym. Marcell, Zosim.]
19 Nay, doth not
Sixtus Quintus [Sixtus V. præfat. fixa Bibliis.] confess that certain Catholics (he meaneth certain of his own side) were in such a humour of translating the Scriptures into
Latin, that Satan taking occasion by them, though they thought of no such matter, did strive what he could, out of so uncertain and manifold a variety of translations, so to mingle all things, that nothing might seem to be left certain and firm in them, etc.?
• 20 Nay, further, did not the same
Sixtus ordain by an inviolable decree, and that with the counsel and consent of his cardinals, that the
Latin edition of the Old and New Testament, which the Council of
Trent would have to be authentic, is the same without controversy which he then set forth, being diligently corrected and printed in the printing-house of
Vatican? Thus
Sixtus in his preface before his Bible.
• 21 And yet
Clement the Eighth his immediate successor, publisheth another edition of the Bible, containing in it infinite differences from that of
Sixtus, (and many of them weighty and material) and yet this must be authentic by all means.
• 22 What is to have the
faith of our glorious Lord JESUS CHRIST, with Yea and Nay, if this be not?
Of course, the “faith of” = (even) “believed
in” . That’s simple.