I have read in the word that a birth should be mourned and death celebrated when a person is a believer. But I can’t remember where. Can you tell me what book, chapter and verse to find this?
I can find no such verse. There would be no reason to mourn a birth, since this is the way God has provided for us to follow his commandment to “be fruitful and multiply.” (Genesis 1:28) Why mourn obedience to a command of God?
While the death of a believer is actually to their benefit, it is still natural to mourn death. Even Jesus mourned when Lazarus died, even knowing that he was going to raise him from the dead. (John 11:33-35) While in a sense we celebrate the death of a believer, we should also allow those who loved him to go through the natural and normal grieving process.