God himself had characterized the world he created as “very good” (Genesis 1:31). He told the human race that the wages of sin was death (Genesis 2:17; Romans 6:23), and spelled out the suffering that would climax in death (Genesis 3:15–19). Pain and sores are never God’s fault but always the penalty humanity has brought into their lives through their rebellion.
The gospel is that God sent his Son to take on himself the “pain and sores” sinners deserve (Isaiah 53:4). But those who refuse to accept that (their own) sin is the cause of life’s “pain and sores” will not repent of their sins and embrace Jesus Christ; instead, they blame God for life’s trials. That’s the hardness and stubbornness John is describing here.
11 and cursed the God of heaven for their pain and sores. They did not repent of their deeds.