Because Rom 6:6, Gal 5:24, and 2 Cor 5:17 say so.
Rom 6:6..."Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin."
Gal 5:24..."And they that are Christ's have crucified the flesh with the affections and lusts."
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