There's only one -- to deny the Holy Spirit. "But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation" (Mark 3:29).
This took me 60 years to understand.
But you have to understand what the Holy Spirit is. The Holy Ghost or spirit is the True Self. When we are "in spirit," we are walking with God as His child, made in His image -- non-physical, spiritual and timeless source of creation.
Forgiveness is the repair we all need. If we deny that our True Self exists, then it can never be repaired. In other words, we would be preventing our own forgiveness by "hiding" that which needs to be repaired.
And the only way to awaken the spirit is to let go of all self-concern (fear, hate, anger, distrust, jealousy, resentment, guilt, depression, etc), and to put our attention on loving others.