What is the biggest sin for a Christian?
Christianity and Judaism
In contrast to the Jewish view of being morally balanced, Original Sin refers to the idea that the sin of Adam and Eve's disobedience (sin "at the origin") has passed on a spiritual heritage, so to speak. Christians teach that human beings inherit a corrupted or damaged human nature in which the tendency to do bad is greater than it would have been otherwise, so much so that human nature would not be capable now of participating in the afterlife with God. This is not a matter of being "guilty" of anything; each person is only personally guilty of their own actual sins. However, this understanding of original sin is what lies behind the Christian emphasis on the need for spiritual salvation from a spiritual Saviour, who can forgive and set aside sin even though humans are not inherently pure and worthy of such salvation. Paul the Apostle in Romans and I Corinthians placed special emphasis on this doctrine, and stressed that belief in Jesus would allow Christians to overcome death and attain salvation in the hereafter.
Western Christianity
Although "original sin" can be taken to mean the sin that Adam committed, it is usually understood as a consequence of the first sin, the hereditary stain with which we are born on account of our origin or descent from Adam.
With the exception of tendencies such as Pelagianism, Western Christianity is thought to hold this doctrine, which was championed especially by Saint Augustine, who wrote: "The deliberate sin of the first man is the cause of original sin" (De nupt. et concup., II, xxvi, 43).