'cheap grace' may be found in believers and in unbelievers, truly saved and not saved - it may be a sin unto death, and it may be a belief someone holds until they die because they refuse the truth and continually reject the Messiah Yeshua. it may or may not refer to the multitudes of pagans who were officially made part of the church against their will, i don't know- never entered my mind before the last minute.
concerning the sin unto death, there are several in Scripture, and many perhaps may be seen in our lifetime. the best truthful, tested, Biblically accurate summary I've ever seen is in
'Overcoming Death' by Watchman Nee - (freely distributed)
"The Word has not spelled out concretely what this sin is. It only assures us that such a sin is possible. From the Scripture records we understand that this sin varies according to people. A particular sin for some is mortal, yet to another person it may not be a sin unto death, and vice versa. This is because of differences in grace received, light accepted, and position attained among different believers.
While the Bible never identifies this sin, we can nevertheless observe that any sin which results in death constitutes a mortal one. The people of Israel committed such a sin at Kadesh (Num. 13.25-14.12). Although they had tempted the Lord many times before (14.22), He always simply forgave them. But this time, though He still forgave them after they refused to enter Canaan, He additional caused their bodies to fall in death in the wilderness (14.32).
At the waters of Meribah Moses was provoked to speak “words that were rash” (Ps. 106.33) : this was his “mortal sin”: he died outside Canaan. Aaron committed the same offense as Moses and he likewise was forbidden to enter the holy land (Num. 20.24). The man of God who journeyed from Judah to Bethel disobeyed the commandment of the Lord with regard to eating and drinking; in so doing he committed his mortal sin (1 Kings 13.21-22). In the New Testament we learn how Ananias and Sapphira were punished with death because they committed what for them was their mortal sin, because they attempted to lie to the Holy Spirit by keeping back part of the proceeds from their land (Acts 5). The man in Corinth who lived with his father’s wife was guilty too of this kind of sin, forcing the Apostle Paul to pronounce judgment by telling those at Corinth “to deliver this man to Satan for the destruction of the flesh” (1 Cor. 5.5). Not a few of the brethren in Corinth died because they were guilty of profaning the body and blood of the Lord (1 Cor. 11.27,30). They had committed the sin unto death.
To overcome mortality we must persistently overcome sin, for the former results from the latter. If we wish to live till our days are accomplished or till the Lord returns, we should be careful not to sin. Negligence in this has driven many to the grave prematurely. The mortal sin is not any particular terrifying transgression, because it is nowhere fixed or specified. Such a sin as fornication, of which the Corinthians were guilty, may be counted as mortal; but so too may rash words such as Moses uttered become a sin unto death (for note how the Scriptures characterized Moses: “now the man Moses was very meek, more than all men that were on the face of the earth” Num. 12.3; therefore no sin could be overlooked in this man’s life).
Now is the day of grace. God is full of grace. So let our hearts be comforted. Do not allow Satan to accuse you, hinting that you have committed the mortal sin and hence must die. Although the Bible does not encourage us to pray for others who have sinned this mortal sin, God will forgive us if we judge ourselves and genuinely repent. The man in 2 Corinthians 2.6-7 is believed by many to be that very one who had lived with his father’s wife. In 1 Corinthians 11.30-32 we also are reminded that even though we may have committed the sin unto death, we can nevertheless escape death if we judge ourselves truly. Therefore never permit any sin to reign in your body lest it become your mortal sin. Our flesh can be weakened, yet we must never lose the heart of self-judgment. We must judge our sin without mercy."