Preaching should cover every instance of what it means to sin, and what to do about sin. It should NOT take one sin and preach on that to the devaluation of everything else. The Bible lists certain sins. All that we do against God will get us eternal death. The book of Revelation lists certain things that will keep people separated from God. To lie is as serious as any sexual sin. It brings eternal death.
To be a coward is also considered spiritually evil and will end in eternal death. We need to take every instance of sin listed and talked about in the Bible and treat it all as equally evil in God’s sight. He hates it all, and will eventually deal with it all.
Preaching should be balanced. There’s every need to teach about sin, but there is also every need to preach about forgiveness. There is forgiveness for everything we do. There is only ONE SIN that will NEVER be forgiven. That is the sin against the Holy Spirit. Jesus said that this sin is the one which can never be forgiven, either in this life, or in the next life. Every single other sin will be forgiven us if we ask the Lord to help us and teach us to hate our own sin and to turn from it.
That’s His promise. If we turn from our sin, we are forsaking it for His sake. That’s repentance, and it leads to eternal life.