What is the biggest sin for a Christian?
Eternal sin
In Christian hamartiology, eternal sins, unforgivable sins, or unpardonable sins are sins which will not be forgiven by God. One eternal or unforgivable sin (blasphemy against the Holy Spirit) is specified in several passages of the Synoptic Gospels, including , , and .
Christian views on sin
Roman Catholic doctrine also sees sin as being twofold: Sin is, at once, any evil or immoral "action" which infracts God's law and the inevitable consequences, the "state of being" that comes about by committing the sinful action. Sin can and does alienate a person both from God and the community. Hence, the Catholic Church's insistence on reconciliation with both God and the Church itself.