Paul knew that the outward man or woman was perishing, day by day, and as he said “was wasting away” but the most important work was being done inwardly by the Spirit of God and this was the renewal that each believer in Christ is undergoing, day by day. God’s Spirit works to convict us of sin, He works to make us more like Christ, and our sanctification is a lifelong process and even though we might be falling apart outwardly, what is eternal is what is inside and that’s the only thing that counts. We are new creations in Christ (2 Cor 5:17) even though the outward appearance is decaying, so that’s why Paul didn’t want us to lose heart.
Now there are varieties of gifts, but the same Spirit; and there are varieties of service, but the same Lord; and there are varieties of activities, but it is the same God who empowers them all in everyone. To each is given the manifestation of the Spirit for the common good. For to one is given through the Spirit the utterance of wisdom, and to another the utterance of knowledge according to the same Spirit, ... Scripture Verse Gift