I think...the question is not as simple about believing or not believing "the truth" especially the authority of the FDS. I believe their is a degree of sincerity in all JWs including the Gov Body, or most of its members. But I never cease to be amazed at the ability of the human mind to delude itself and suppress doubts. Many continue in their ways by not thinking too deeply about things, JWs find this easier because of all the buzz words - like apostate - that automatically trigger a subconscious response.
The local PO who I was studying with admitted, with no witnesses present, in my home, that the FDS does not receive any information from God. I asked why rank & file JWs believed they did, but he would not answer. He stopped visiting me and I was dumped from Bible study - so he must know its all nonsense.
Then I asked myself why he did not want to discover the real truth - which is my own quest. This PO had become a JW at 12 and the higher he went in the hierarchy, the more status he gained, and stood to lose. Moreover, "where else would he go?" That was the only life he knew. He could not allow himself to face up to reality after all this time. The higher people go, the more they know its false, but by then they have too much to lose - esecially status. Just my observations.