The Ante-Nicene Fathers still had recognizable portions of the Church of Jesus Christ in their midst, even though the Prophets and Apostles had been killed. They knew that they had lost the revelators, or revelations from the Spirit, but they still believed many of the things that Christ and the Apostles had taught us in the Bible, including the doctrines of deification and pre-mortal life.
After the Nicene Creed and the associated councils, much of the Gospel of Jesus Christ and the Church of Jesus Christ were voted out of existence. The doctrines of deification and pre-mortal life were voted out of existence. The true nature of God was destroyed. The Post-Nicene Church literally banned the doctrine of a pre-mortal life; and, that doctrine had to be restored to us through Joseph Smith. Salvation for the Dead was destroyed. They corrupted and perverted baptism into something that was nothing like the Baptism that John the Baptist offered. The doctrine of a literal physical resurrection was lost. The three degrees of glory that Paul spoke of were lost. The good stuff was simply voted out of existence by the Nicene Creed and replaced by Greek philosophy.
Therefore, a lot of what remained was changed and was then no longer in harmony with the Bible or the Gospel of Christ among the Post-Nicene Fathers. Since they no longer had the spirit of revelation or the gift of the Holy Ghost, they were just making things up and voting on them as they went along. Much of what they came up with had nothing to do with the Gospel of Jesus Christ or the Church of Jesus Christ. They were just the philosophies of men, rather than revelations from God.
This website contains a free version of the writings of the Ante-Nicene Fathers:
Some people consider the writings of the Ante-Nicene Fathers to be the greatest gift that we ever received from the Catholic Church. Obviously, those writings were hid from the world for over a millennium, and we should be grateful that they were preserved.
The writings of the Post-Nicene Fathers are there also. A comparison between the two can be quite enlightening.
Tad Callister's book discusses what was lost or voted out of existence by the Nicene Creed and subsequent creeds:
http://ldssoul.com/wp-content/uploads/2 ... lister.pdf