Since when did Eru forbid men from going to the Undying Lands?

Question

I read in The Silmarillion that the Valar forbade the Númenóreans going to the Undying Lands. Was this rule made by Eru himself and did he love Elves more than Men? Both Elves and Men were the Children of Ilúvatar.

I found it hard to explain to people that God abandoned Men in Middle-earth by removing Aman from Arda and the fact that Valar did not go to Middle-earth to destroy Sauron like they did to Morgoth.

Edit: I found some evidence that Mandos really hated Men in the Undying Lands, when Eärendil went to the Undying Lands for the Valar's help against Morgoth:

It is told among the Elves that after Eärendil had departed, seeking Elwing his wife, Mandos spoke concerning his fate; and he said: 'Shall mortal Man step living upon the undying lands and yet live?' But Ulmo said:' For this he was born into the world. And say unto me: whether is he Eärendil Tuor's son of the line of Hador, or the son of Idril, Turgon's daughter, of the Elven-house of Finwë? And Mandos answered: 'Equally the Noldor, who went wilfully into exile, may not return hither.'

Answer 1

For a start, we have no evidence of any rules laid down by Eru, or indeed of any communication between Eru and the Valar apart from when the Valar "laid down their stewardship" and Eru made the world round. There is certainly nothing to indicate that the Ban was commanded by Eru; it seems plain it is a rule of the Valar only.

Secondly, I don't know why you think the Ban means the Valar "abandoned" Men. That's not implied at all. The Valar, like the Elves, are bound to the world until the End. But Men leave the world when they die, which - from the point of view of the others - means that they are getting a Gift. The Valar know not to interfere with that. It's the opposite of abandoning them.

And finally, it's not true that the Valar did nothing to defeat Sauron. They send the Istari (Wizards), and it was mainly thanks to Gandalf that Sauron was defeated.

Answer 2

There is no evidence of Eru specifically saying that men were not allowed into the Undying lands. But the name "Undying lands" is in itself very suggestive. We do know however that a select few mortals did in fact go to Aman (Eärendil, Bilbo, Frodo, Sam and Gimli) so it is not that they physically couldn't.

EDIT: As for your quote from Mandos the Noldor were not to return because of the "Doom of the Noldor/Mandos"

"Tears unnumbered ye shall shed; and the Valar will fence Valinor against you, and shut you out, so that not even the echo of your lamentation shall pass over the mountains. On the House of Fëanor the wrath of the Valar lieth from the West unto the uttermost East, and upon all that will follow them it shall be laid also.

Again, Mandos objections to Eärendil are on the grounds that he came to Aman after it had been hidden from the world. Ulmo says that he is half Noldor, half Edain to which Mandos replies that the Noldor are banished from Aman as well.

Answer 3

An important thing to keep in mind is that the Valar had, by this time, realized the mistake they had made by attempting to protect the Elves by secreting them away to a guarded paradise. Some of the Valar had already predicted, or objected, to inviting the three kindreds of Elves to Valinor, saying that it was indeed Eru's plan for them to inherit Middle-earth and learn to survive there. They were outweighed by the fear that the destruction of Utumno ("Hell"), Melkor's original fortress under the Earth, would accidentally kill the Elves before they awakened. Once imprisoned and then released, Melkor deceived the Elves into rebelling against their helicopter parents, the Valar.

Fëanor unwittingly repeated Morgoth's lies, arguing in his famous speech that the Elves were indeed meant to inherit Middle-earth and rule over men, but that the Valar were jealous of the gift of Elves (even as it is argued by others they were of the Gift of Men) to live out their lives independently of the will of Eru, and that the Elves had been brought to Valinor in an effort to keep them in their place. Using this rhetoric Fëanor led the first Kinslaying on the shores of the Undying Lands, and Mandos knew what else was prophesied to happen. Not wanting to make the same mistake again, the Valar were understandably reluctant to let anyone else into the Undying Lands, lest they covet what they could not have.