What is a mortal sin?
It is a sin unto death (1John 5:16). Under the law, murder was a sin unto death. In the case of Ananias and Sapphira lying unto the Holy Spirit was a sin unto death. In 1Cor 11:30 Paul speaks of some who sin and are sickly, and some even died. That was a sin unto death. Mortal sin was never spoken of a Gentile. It was to those under the law. Today the law is not in effect, so we see no examples as above among Christians.
