Verse one goes with chapter six. It creates a paradigm for Christian living. "Having these promises. . ." To be a Christian is to be a person of the promise. Christianity starts with a word from God that is either true or not; he is either good for the promise or he is lying. Responding to the promise by faith (that's what you do with promises -- you believe them), we purify ourselves inbody and spirit and take aim for holiness in the fear of God.
Paul's earlier letter started them grieving but the grief worked repentance, so it was not useless or without profit. Grief is not wasted if it humbles us and prompts response to God. In the end, their grief let to repentance which brought great joy! The apostle Paul had confidence in them because they traveled this road of grief, repentance, and joy. That pathway leads to spiritual confidence.