Every part of human nature is deeply damaged by the Fall. We are born with a powerful bias toward sin. God commands us to confess our sin and turn from it to him as our Savior and Lord. We require the new spiritual birth which the Holy Spirit gives. Any who refuse God’s command and offer of life will suffer everlasting separation from him in hell.
Why is it necessary that we believe this? Every person, no matter how moral and good, will fall short of God’s standard because of a sinful nature (we have all done things we regret). Therefore being ‘good’ is not ‘good enough’. Being religious doesn’t make us acceptable to God (many have tried that). Acceptance comes through a ‘right relationship’ with Him. This is achieved by confessing our sins, seeking to live according to God’s ways. This is actually impossible humanly speaking, but God the Holy Spirit works in our lives so that we can become ‘new people’ and serve him. Many reject God or want Him on their ‘terms’. We can’t bargain or argue with God. He has made a way for us to be forgiven. If we reject this, we reject God. Sadly, the alternative is an eternal hell without God.