For just as the sufferings of Christ flow over into our lives, so also through Christ our comfort overflows. — 2 Cor. 1:5
This passage in 2 Corinthians 1, verses 3-7, is perfect for using when trying to comfort someone in mourning. It stresses the fact that Christ suffers along with us. While the passage clearly refers to sufferings and persecution for spreading the Gospel, I believe the message can apply to others in suffering.
May be used for things like funerals.
Is. 57:1 The righteous perish, and no one ponders it in his heart; devout men are taken away, and no one understands that the righteous are taken away to be spared from evil. 2 Those who walk uprightly enter into peace; they find rest as they lie in death.