Why would anyone, who has crucified the flesh, with the affections and lusts, (Gal 5:24), still be fighting something that is long gone ?
Because it isn’t long gone. That is taught nowhere in Scripture. Nowhere is it taught that we cannot sin or do not sin.
What is taught is that we are to continually confess our sins for forgiveness (1 John 1:9), as well as try to be holy with the help of the Holy Spirit. As we do so, we grow in sanctification and sin less and less. But we are
never told we will be completely free of sin in this life. Remember, John says that any “believer” who claims to be without sin is self-deceived, doesn’t have the truth, makes God a liar, and doesn’t have his word in them (1 John 1:8, 10).
This is what Paul means when he is talking about the flesh crucified:
Rom 6:6 We know that our old self was crucified with him in order that the body of sin might be brought to nothing, so that we would no longer be enslaved to sin.
Rom 6:7 For one who has died
has been set free from sin.
Rom 6:8 Now if we have died with Christ, we believe that we will also live with him.
Rom 6:9 We know that Christ, being raised from the dead, will never die again; death no longer has dominion over him.
Rom 6:10 For the death he died he died to sin, once for all, but the life he lives he lives to God.
Rom 6:11 So you also must consider yourselves dead to sin and alive to God in Christ Jesus.
Rom 6:12 Let not sin therefore reign in your mortal body, to make you obey its passions.
Rom 6:13 Do not present your members to sin as instruments for unrighteousness,
but present yourselves to God as those who have been brought from death to life, and your members to God as instruments for righteousness.
Rom 6:14 For sin will have no dominion over you, since you are not under law but under grace.
Rom 6:15 What then?
Are we to sin because we are not under law but under grace? By no means!
Rom 6:16 Do you not know that if you present yourselves to anyone as obedient slaves, you are slaves of the one whom you obey, either of sin, which leads to death,
or of obedience, which leads to righteousness?
Rom 6:17 But thanks be to God, that you who were once slaves of sin have become obedient from the heart to the standard of teaching to which you were committed,
Rom 6:18 and,
having been set free from sin, have become slaves of righteousness.
Rom 6:19 I am speaking in human terms,
because of your natural limitations. For just as you once presented your members as slaves to impurity and to lawlessness leading to more lawlessness,
so now present your members as slaves to righteousness leading to sanctification. (ESV)
Notice what Paul doesn't say--that believers are dead to sin; that they cannot sin anymore. What he does say is that because of the death and resurrection of Christ, we are "no longer. . . enslaved to sin," have "been set free from sin," and "sin will no longer have dominion over [us]."
Does that mean we cannot sin anymore? No, and the rest of the context makes this clear. We are to "consider [our]selves dead to sin;" we are to "Let not sin reign in [our] mortal bod[ies], to make [us] obey its passions;" we are not to "present [our] members to sin as instruments for unrighteousness;" we are not to "sin because we are not under law but under grace." So,
Paul gives specific commands of what believers are
not to do in regards to sin, which is meaningless if we cannot sin. More that than,
Paul gives specific commands of what believers
are to do in regards to righteousness, which also is meaningless if we cannot sin:
"consider yourselves dead to sin and alive to God in Christ Jesus;" "present yourselves to God as those who have been brought from death to life, and your members to God as instruments for righteousness;" "now present your members as slaves to righteousness leading to sanctification."
It's worth noting that in v. 19, the ESV's rendering of 'because of your natural limitations," is literally in the Greek, "on account of the weakness of the flesh of you." This is why other versions say:
Rom 6:19a (I am speaking in human terms because of the weakness of your flesh.) (LEB)
Rom 6:19a I put this in human terms because you are weak in your natural selves. (NIV)
Rom 6:19a I am speaking in human terms because of the weakness of your flesh. (NASB)
Rom 6:19a I speak after the manner of men because of the infirmity of your flesh: (KJV)
They all say the same thing as the ESV, but are closer to the Greek. We still fight sin in the flesh but are to "present [our] members as slaves to righteousness leading to sanctification." That is the Christian life; this is warfare, not retirement living. And all that is what Paul means by having our flesh crucified and having died with Christ. We have been set free from slavery to sin, but still fight against it as we are now to be slaves to righteousness, leading to sanctification.