What happens if you have an infection and you don’t get an antibiotic for it? It will spread and be very dangerous to your health. The same is true with sin. If there is a sin in our life it usually starts small and gets bigger and bigger until it is full blown. None of us sin-free because we are human. That is why Christ died for our sins. The only way to wash them away is confess them to the Lord and ask for forgiveness. Our flesh is sinful but if our spirit is the Holy Spirit we can overcome these downfalls. But, without Christ and constant time in the Word of God the sin will grow and take over our life.
For I know that good itself does not dwell in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out