The Bible is referenced a few times in the Buffyverse. In the episode Angel, the Master says, “out of the mouths of babes”, which is from Psalms 8:2. Absalom, the religious vampire in When She Was Bad, was named after a character in the Bible. Absalom was the third son of King David. He turned against his father and challenged him for the kingdom of Israel. The story is told in the second book of Samuel (2 Samuel 13:20 - 19:10).
The title of the episode Faith, Hope and Trick is from the Bible verse 1 Corinthians 13:13: “And now these three remain: faith, hope and love. But the greatest of these is love.” Love has been replaced by Trick.
In Hush, as Buffy and Willow are first walking through town after their voices have been stolen, they walk past a group of Christians reading from the bible. The verse they’re reading is Revelations 15:1: “Then I saw another portent in heaven, great and wonderful: seven angels with seven plagues, which are the last, for in them the wrath of God is ended.”
In Never Leave Me, Quentin Travers quotes Proverbs 24:6: “Proverbs 24:6. O, by wise council, you shall make your war”.
In Dirty Girls, Caleb says to Faith, “Well you’re the other one. The Cain to her Abel. No offence to Cain, of course.” In the Bible, Cain and Abel were the sons of Adam and Eve. Cain murdered Abel out of jealousy.