The gospel of Thomas was written by gnostics who denied the divinity of Christ. Therefore, it was excluded from the canon. The gnostics were proven to be frauds then and they still are now.
Skiff, we as Christians believe that the Bible was indeed penned by human authors. But we believe they were all under the divine influence of the Holy Spirit. Throughout the entire process that ultimately led to the Bible we have today, we believe God was there guiding it along. An omnipotent, omniscient being is capable of such guidance, I think.
One wonders why god would feel the need to exclude gospels and to have multiple Councils of deliberations about the book.