Why can’t a true Christian fight for any “earthly thing” but rather are told to ‘lay down our lives instead’?

The Reason: As a testament of obedience and Love toward God first, and next, to our fellow men including our enemies too (“no” hypocrisy in Love either).

If we’re “not” allowed to fight for the King Himself, we will “not” fight for anyone else either; “no”, not even for our own lives should the day come (nothing else is “worthy” compared to Him).

An ancient command by the King Himself lays buried in us (God Willing, may we “not” betray Him for ‘any reason’):

“Jesus answered, “My kingdom is not of this world. If My kingdom were of this world, then My servants would be fighting so that I would not be handed over to the Jews; but as it is, My kingdom is not of this realm.” (John 18:36)

P/S:

None have beaten us because we did not fight back. We let them win instead since we’re commanded to demonstrate Love to God by showing it toward our fellow men which includes our enemies too.

“Let love be without hypocrisy. Abhor what is evil; cling to what is good.” – apostle Paul (Romans12:9)

If the mightiest armies of heaven are silent about it, so will we let it be.

He stopped them (“more” than 12 legions of angels) and any of us as it’s written:

“Then Jesus said to him, “Put your sword back into its place; for all those who take up the sword shall perish by the sword.

“Or do you think that I cannot appeal to My Father, and He will at once put at My disposal more than twelve legions of angels?” (Matthew 26:52 – 53)

Scripture must be fulfilled (our role as true believers is to “obey” His Command to “Love” and “not” fight nor resist even the evil person as He commanded):

“How then will the Scriptures be fulfilled, which say that it must happen this way?” (Matthew 26:54)