Seeing the title, I thought this might be your angle.
It's God's angle by plain Scripture. Unless someone can show how it's not. But how is that possible, when it's just repeating what's written with normal grammar and comprehension?
Rev{3:15} I know thy works, that thou art neither cold nor hot: I would thou wert cold or hot. {3:16} So then because thou art lukewarm, and neither cold nor hot, I will spue thee out of my mouth.
Jesus judges lukewarm faith by works. Can't get any clearer than that.
Neh 8:8So they read in the book in the law of God distinctly, and gave the sense, and caused them to understand the reading.
In fact, it's not just the faith, but the person that Jesus judges lukewarm and spits out of His body.
How do you deal with the corrupt flesh,
I deal with naturally mortal flesh the same as every creature on earth. Corruption of the flesh in Scripture is natural mortality as created by Christ.
I don't deal with corruption of the heart, which is lust and sins of the world with the flesh. I repent of it for Jesus' sake.
Trying to deal with continued sinning against Jesus Christ, is the hard work of the world.
Pro 13:15Good understanding giveth favour: but the way of transgressors is hard.
i.e., does it exist for for all Christians,
The lie of the natural flesh created by Christ being sinful, which God creates and gives on earth for the good, not the evil.
Rev 4:11Thou art worthy, O Lord, to receive glory and honour and power: for thou hast created all things, and for thy pleasure they are and were created.
That lie against Christ the Maker of all flesh, only exists in the vain imaginations of some Christians naming Christ, as a ready excuse for courrpt living. It's the childishness of trying to blame the body for what we do with the body, or at least hold the flesh partly responsible.
Responsible children know better than that. Only in the world of doctrinal corruption, do some try to make their bodies the source for their bad manners.
1Co 15:33Be not deceived: evil communications corrupt good manners.
lusting in adversarial opposition to that of Spirit (as stated in Ga 5:17)?
Lusting with the world is the same for all corrupt sinners of the world, whether naming Christ or not.
It's the result of not repenting for Jesus' sake to recieve the promise of salvation from the lust and sins of the world:
2Pe 1:3According as his divine power hath given unto us all things that pertain unto life and godliness, through the knowledge of him that hath called us to glory and virtue.
Whereby are given unto us exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust.
If so, to avoid being lukewarm, must a Christian always walk in the Spirit?
My. Wouldn't that be horrible. No more lusting and sinning? God forbid.
Of course, with the hard of heart by sin, walking with Jesus is shunned in this life and the next.
Act 19:9But when divers were hardened, and believed not, but spake evil of that way before the multitude, he departed from them, and separated the disciples, disputing daily in the school of one Tyrannus.
Heb 3:13But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.
Why would anyone want to walk with the Lord in white forever, if not here and now on earth.
Rev{3:4} Thou hast a few names even in Sardis which have not defiled their garments; and they shall walk with me in white: for they are worthy.
If not, where does a true Christian's faults come from?
There are true Christian saints:
Jde 1:20 But ye, beloved, building up yourselves on your most holy faith, praying in the Holy Ghost, Keep yourselves in the love of God, looking for the mercy of our Lord Jesus Christ unto eternal life.
Jde 1:24Now unto him that is able to keep you from falling, and to present you faultless before the presence of his glory with exceeding joy, To the only wise God our Saviour, be glory and majesty, dominion and power, both now and ever. Amen.
And true Christian sinners:
Mar 4:19And the cares of this world, and the deceitfulness of riches, and the lusts of other things entering in, choke the word, and it becometh unfruitful.
Jesus judges the difference by our works, whether good, evil, or worst of all lukewarm.