The One True God is Allah

Allah says,

إِنَّ إِلَهَكُمْ لَوَاحِدٌ ﴿٤﴾

رَبُّ السَّمَاوَاتِ وَالْأَرْضِ ...

Verily, your God is indeed One, Lord of the heavens and the earth,

This is the One by Whom the oath is sworn, stating that there is no God worthy of worship but He, Lord of the heavens and the earth,

... وَمَا بَيْنَهُمَا ...

and all that is between them,

means, of created beings.

... وَرَبُّ الْمَشَارِقِ ﴿٥﴾

and Lord of every point of the sun's risings.

means, He is the Sovereign Who is controlling His creation by subjugating it and all that is in it of stars, planets and heavenly bodies which appear from the east and set in the west.

Mentioning the east is sufficient and there is no need for the west to be mentioned too, because it is implied in what is said.

This has also been stated clearly elsewhere, in the Ayat:

فَلاَ أُقْسِمُ بِرَبِّ الْمَشَـرِقِ وَالْمَغَـرِبِ إِنَّا لَقَـدِرُونَ

So I swear by the Lord of all the points of sunrise and sunset in the east and the west that surely We are able. (70:40)

رَبُّ الْمَشْرِقَيْنِ وَرَبُّ الْمَغْرِبَيْنِ

(He is) the Lord of the two easts and the Lord of the two wests. (55:17)

which refers to the rising and setting points of the sun and the moon in both winter and summer.