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A sin unto death of a believer who has eternal life with God because of Jesus only has a cessation of their physical life.
MOSES
Numbers 20:12 . . . Because ye believed me not.
Deuteronomy 32:50 Die in the mount whither thou goest up.
Deuteronomy 32:51 Because ye trespassed against me among the children of Israel.
Notice that Moses sin was unbelief. He was killed for it just as those destroyed in Jude :5 that believed not.
ANANIAS, with SAPPHIRA lied unto God
Act 5:1 But a certain man named Ananias, with Sapphira his wife, sold a possession,
Act 5:2 And kept back part of the price, his wife also being privy to it,
They both died as the result.