Specifically, according to Paul Sin seizes opportunity through the Law because of the weakness of the flesh. As Paul writes in Romans 8.6-7:
For to set the mind on the flesh is death, but to set the mind on the Spirit is life and peace. For the mind that is set on the flesh is hostile to God, for it does not submit to God's law; indeed, it cannot.In Chapter 7 Paul gives a vivid description about how the flesh is unable, under the power of Sin, to obey God's Law:
Romans 7.14-15, 18Notice the key theme: Incapacity. The flesh does not submit to God's law; indeed it cannot. I have the desire to do what is right, but not the ability to carry it out.
For we know that the law is spiritual, but I am of the flesh, sold under sin. For I do not understand my own actions. For I do not do what I want, but I do the very thing I hate. For I know that nothing good dwells in me, that is, in my flesh. For I have the desire to do what is right, but not the ability to carry it out.
The deep issue for Paul is human incapacity and weakness, our congenital inability to carry out God's good, righteous and holy commands.
To be clear, Paul isn't preaching "total depravity." In the picture Paul is painting we both know and desire to do the right things. Deep down, we are good people. The problem is that we're too weak to be the good people we desire to be. The issue isn't wickedness, but weakness.
Overcoming this incapacity, then, is the main point of salvation. And according to Paul, our fleshly incapacity is overcome by the power of the Spirit: "For if you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live." (Romans 8.13)
What's interesting here is how this reading of Paul isn't new or modern. This is the primary way the church fathers understood salvation. Specifically, salvation is less about the forgiveness of sin than the Spirit healing human weakness.
For example, Athanasius describes in On the Incarnation how Adam's sin returned humanity to a mortal, animal existence. In the Garden, when we had communion with God, we had been protected from death and corruption: "Because of the Word present in them, even natural corruption did not come near them." But after the Fall, we fell into a weakened mortal state: "When this happened, human beings died and corruption thenceforth prevailed against them." Under the sway of death, sin began to dominate human existence the whole affair tipping toward madness, violence, and darkness. The Image of God began slipping away from us: "For these reasons, then, with death holding greater sway and corruption remaining fast against human beings, the race of humans was perishing, and the human being, made rational and in the image, was disappearing, and the work made by God was being obliterated."
The human being was disappearing. That was the problem. The Image of God in us was being slowly obliterated.
So as we see in Athanasius, the issue isn't really about our need for God to forgive our sins. The problem was that, separated from God's life, the entire human project was falling into darkness and chaos. The human being was disappearing, leaving only beasts upon the earth. Sure, God needs to forgive us. But God needs to do something more drastic and dramatic to keep the cosmos from tipping over into death and dissolution, to save and secure the Image of God that was fading from the world.
And God does this more dramatic and drastic thing by reuniting God's divine nature with human flesh through the Incarnation. In the Incarnation God permanently marks human flesh with His Image. More, through the resurrection of Incarnated flesh, humans were given power over death and corruption.
The key idea here for Athanasius, and for Paul in Romans, is that salvation is fundamentally about power, a power human flesh lacks when separated from God's divine life. And for Paul, it's the gift of the Spirit that gives us this power. The Spirit is our tether, our umbilical cord, to God's life.
So for Paul, the gospel message isn't primarily about "the forgiveness of sins." The Good News is fundamentally about reunion and participation in the Divine Life, the power of the Spirit to overcome our weakness and incapacity in the face of Sin and Death:
If the Spirit of him who raised Jesus from the dead dwells in you, he who raised Christ Jesus from the dead will also give life to your mortal bodies through his Spirit who dwells in you.