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.
Then God said, “Let us make man in our image, after our likeness. And let them have dominion over the fish of the sea and over the birds of the heavens and over the livestock and over all the earth and over every creeping thing that creeps on the earth.” So God created man in his own image, in the image of God he created him; male and female he created them. Scripture Images