God heals the believer because of His Covenant of Healing. God heals the unbeliever because of His Mercy.
Read: Matthew 5:44-45
44 But I say, love your enemies![a] Pray for those who persecute you! 45 In that way, you will be acting as true children of your Father in heaven. For he gives his sunlight to both the evil and the good, and he sends rain on the just and the unjust alike.