2. Background of the poem
Tolkien wrote in Silmarillion, Ainulindale, The Music of the Ainur:
There was Eru, the One, who in Arda is called Iluvatar; and he made first the Ainur, the Holy Ones, that were the offspring of his thought, and they were with him before else was made. And he spoke to them, propounding to them themes of music; and they sang before him, and he was glad.
… But when they come to the Void, Iluvatar said to them: ‘Behold your Music !’ And he showed to them a vision, giving to them sight where before was only hearing; and they saw a new World made visible before them, and it was globed amid the Void, and it was sustained therein, but it was not of it.
This poem uses pantheism with the notion of an ultimate source of all. We can not know what God is, we can only feel it through our small minds. Eru is one description of it.
Tolkien, Silmarillion, Ainulindale, The Music of the Ainur:
For the Children of Iluvatar were conceived by him alone; and they came with the third theme, …, and none of the Ainur had part in their making.
What is known of Eru in Gondor ? The Valar had contact with the Elves, thus the Elves and from them the Edain and the Dunedain should know of Eru the Greatest Being, at least dimly. Melian, who served in Valinor Vana and Este, lived hundreds of years with Elves. Gandalf, Saruman, Radagast and two other Istari are emissaries of the Valar in Middle Earth. Moreover, the Dunedain of Gondor are the descendants of the Numenoreans who worshiped Eru.
Tolkien, UT, A Description of the Island of Númenor
Near to the centre of the Mittalmar stood the tall mountain called Meneltarma, Pillar of the Heavens, sacred to the worship of Eru Iluvatar . … Thrice only in each year the King spoke, offering prayer for the coming year at the Erukeyerme in the first days of spring, praise of Eru Iluvatar at the Erulaitale in midsummer, and thanksgiving to him at the Eruhantale at the end of autumn. At these times the King ascended the mountain on foot followed by a great concourse of the people, clad in white and garlanded, but silent.
Does Beregond expect a reaction from Eru ? No, only once Eru intervened in Middle Earth which resulted in the Akallabeth (Silmarillion):
Then Manwe upon the Mountain called upon Iluvatar, and for that time the Valar laid down their government of Arda. But Iluvatar showed forth his power, and he changed the fashion of the world;
After this Numenorean catastrophe caused by the hubris of Men neither Eru nor the Valar actively intervene in Middle-Earth in the third age. Thus, Beregond calls Iluvatar just to strengthen his will of defence and to feel more secure about Bergil.
A/N: Bold emphasizements made by me to guide the eye to the worship of Eru in ME by Men.