Paul understood God's mercy. He knew that the things he had done made him the worst of sinners, and yet God saved him. This is the chief characteristic of mercy - giving kindness and help to someone who doesn't deserve it.
Mercy is not something that God possesses, it's something that He is. His personal response to the worst sinner is mercy. Our first reaction is to desire punishment and revenge on those who wrong us. God's desire is to forgive and save us, He has unlimited patience...this is good news for us! Of this Paul says, "Here is a trustworthy saying..."
Mercy is God's redemptive patience in action. He is not willing that any of us should perish, but that we would each be redeemed. Mercy triumphs over judgement.
God's mercy changes how we live our lives. We can have hope for the future in spite of the past, and we can have hope for others in spite of the past. We can forgive and be forgiven.