What is the biggest sin to God?

The Unforgivable Sin

One grave and eternal sin, commonly referred to as blasphemy against the Holy Spirit or the sin leading to death, is highlighted in multiple passages found in the Synoptic Gospels. These include Mark 3:28–29, Matthew 12:31–32, and Luke 12:10. Additionally, this concept is addressed in several other New Testament texts, such as Hebrews 6:4–6, Hebrews 10:26–31, and 1 John 5:16.