The great evil of this heresy is the breaking of a person's contact with reality.
Dare we say that we know of many leaders in the world who believe they are good and godlike, without killing their own wills, without recognizing the Transcendence of God?
Many Catholics choose their own religion which has little to do with the Teaching Magisterium of the Church.
Remember, if one is not orthodox, one cannot even start on the road to perfection.
The number of men and women in the world who are depressed is a symptom of immanentism, as the logical consequence of always following one's own will. The natural consequence of following one's own, subjective way would be depression, because we were not made for ourselves, but for God.
With the rise of mental illness, we could be asking ourselves whether the making of ourselves into gods causes such a disruption of reality, as we are actually denying who we are in the eyes of Our Creator.
The only real peace is in the Will of God. When the early settlers gave thanks to God, they knew who they were. They were children of a Loving, Providential Father, Who had a plan for each one of them. They knew that to be thankful was part of being human.
When we forget who we are and where we are going, we fall into sadness and, even, derangement. Pray for those millions of people in America and elsewhere who have lost the idea of their being rooted in God, in the Trinity, in the truth which is only found in Jesus.
Did you know that the very first Thanksgiving was on September 8, 1565 in St. Augustine, Florida. The Native Americans and Spanish settlers held a feast and the Holy Mass was offered. Look here....