For God has done what the law, weakened by the flesh, could not do. By sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh, (ESV)
God had to do what the law could not do. What does that mean? The Apostle Paul is talking here about the Old Testament Law that the Jews had followed since it was received by Moses himself. The Law was perfect. It was a revelation of God's holy requirements of his called people, the Jews. The Law itself was perfect. Man's flesh was not.
The perfect Law of God was weakened by the flesh. From the time of Moses to the time of the Apostle Paul men and women tried to live according to the Law of God. Every single one of them failed. While some performed better than others no one was perfect in fulfilling the Law of God. What was God's standard in the Law? Absolute flawless perfection.
The Law could not make mankind right with God. It could only show mankind just how unholy he was. Something needed to happen. God stepped into the situation in the form of his perfect son, Jesus. Paul says that Jesus was in the likeness of sinful flesh. This does not mean that Jesus was sinful, but rather that he took on humanity. He was fully God in human form.
Not only did Jesus take on human flesh, but he also took all of mankind's sin on himself. This is why his sacrifice on the cross provides redemption for us. Man's sin was condemned on the back of Jesus at the cross. If it were not for Christ sin would be condemned in our body, which would be our eternal death. God condemns sin in Christ's body, who took our place for punishment.
The Law was perfect, but we are not. Jesus is perfect and took our sin. God took the step toward us in providing the way to be made right with him. If you have not accepted his gift, now would be a perfect time. Ask him to forgive you of your sin, and receive his gift of eternal life.