If we know what a mortal sin is (unto death) we are more likely to steer clear of it and protect our bodies and overcome death.

1 John 5.16 does not point to spiritual death, for the eternal life of God can never be extinquished; nor can this be an allusion to the "second death" since the Lord's sheep cannot perish. It necessarily signifies the death of the body. This is a mortal sin which believers may commit.

1Jo 5:16 If any man see his brother sinning a sin not unto death, he shall ask, and [God] will give him life for them that sin not unto death. There is a sin unto death: not concerning this do I say that he should make request.

If someone is dying because of a sin that which there is no turning from because it is a sin unto death, do not pray for release from. It is a permanent effect upon the body. If it is God's will then to see physical death because of this sin, then why would you pray against God's will?

Num. 13.25-14.12 shows us a mortal sin at Kadesh. Although they tempted the Lord many times before (14.22), He always simply forgave. But this time, He forgave them AND caused their bodies to fall in death in the wilderness (14.32).

Moses spoken rash words (Ps. 106.33) at the waters of Meribah, so this was a mortal sin that caused him to die outside Canaan. Aaron committed the same offense (Num. 20.24). The man of God disobeyed a commandment of the Lord regarding eating and drinking; in so doing committed his mortal sin (1 Kings 13.21-22). In Acts 5 Ananias and Sapphira committed sin which for them was a sin unto death.

Different people have a sin that is mortal while for others it is not because of differences in grace received, light accepted, and positioned attained among different believers.

The man who lived with his father's wife was delivered "to Satan for destruction of the flesh" (1 Cor. 5.5).

Several at Corinth died for profaning the body and the blood of the Lord (1 Cor. 11.27,30). This is a sin unto death also.

This mortal sin is nowhere fixed or specified. The more we glorify God in being meek, the more we are accountable to. Fornication for the Corinthians was counted as mortal, but for Moses, harsh words were also. According to Num. 12.3 Moses was the meekest of of anyone on earth. That would explain why God gave the Word to Him.