The Silmarillion:Ainulindalë

[Cirdan]

"Who is Iluvatar?" said Eriol. "Was he of the Gods?"
"Nay," said Rumil, "that he was not, for he made them. Iluvatar is the Lord of Always who dwells beyond the world; who made it and is not of it or in it, but loves it."

- The Book of Lost Tales

'Lo! After the departure of the Ainur and their vassalage all was quiet for a great age while Iluvatar watched. The all of a sudden he said: "Behold I love the world, and it is a hall of play for Eldar and Men who are may beloved. But when the Eldar come they will be the fairest, and the most lovely of all things by far; and deeper in the knowledge of beauty, and happier than Men. But to Men I will give a new gift, and a greater." Therefore he devised that Men should have a free virtue whereby within the limits of the powers and substances and changes of the world they might fashion and design their life beyond even the original Music of the Ainur that is fate to all things else. This he did that of their operations everything should in shape and deed be completed, and the world fulfilled unto the last and smallest. Lo! Even the Eldar have found to our sorrow that Men have a strange power for good or ill and for turning things despite Gods and Fairies to their mood in the world; so that we say: "Fate may not conquer the Children of Men, but yet are they strangely blind, whereas their joy should be great."

Now Iluvatar knew that Men set amid turmoils of the Ainur would not be ever of a mind to use that gift in harmnoy with his intent, but thereto he said: "These too in their time shall find that all they have done, even the ugliest of deeds and works, redounds at the end only to my glory, and is tributary to the beauty of my world." Yet the Ainursay that the thought of Men is at times a grief even to Iluvatar; wherefore in the giving of that gift of freedom was their envy and amazement, the patience of Iluvatar at its misuse is a matter of the greatest marvelling to both Gods and Fairies. It is however of one with this gift of power that the Children of Men dwell only a short time in the world alive, yet do not perish utterly for ever, whereas the Eldar dwell til the Great End unless they be slain or waste to grief (for to both of these deaths are they sublect), nor doth eld subdue their strength, exceptit may be in ten thousand centuries; and dying they are reborn in their children, so that their number minishes not, nor grows. Yet while the Sons of Menwill after the passing of things of a certainty join in the Second Music of the Ainur, what Iluvatarhas devised for the Eldar beyond the world's end he has not revealed even to the Valar, and Melko has not discovered it'

- The Book of Lost Tales

These two passages, the first before the Music of the Ainur and the latter after, give a somewhat different nature to the tale than the final version. The first passage seams to be a effort to differentiate Iluvatar from the god of Tolkien's own faith. While it is clear that he sought to create a unique story of creation, there is little doubt of the heavy influence on his writing of his own faith.

The second passage is an interesting detail of the nature of Men. Here they are the only beings free from the fates. Yet even they cannot escape the eventual destiny of Middle Earth. This fatalism of the earlier version of the story is quite a bit stronger than the final version. Whether it is an edit of literary content or a change of philosophy to the final version in the Silmarillion is impossible to say. Many of the stories in the Book(s) of Lost Tales are much greater in detail. The stories are also told as a narrative within a larger tale. If you enjoyed the Silmarillon you with like the BoLTs as well. The Tales of Tuor, Gondolin, and Earendil are much greater in detail than those given in the Silmarillon. I highly reccommend the read.