I will quote my favourite verse(s) from the scripture or holy book of Islam, Hinduism, Christianity/Judaism.

Islam (The Quran):

“Allah – there is no deity except Him, the Ever-Living, the Sustainer of [all] existence. Neither drowsiness overtakes Him nor sleep. To Him belongs whatever is in the heavens and whatever is on the earth. Who is it that can intercede with Him except by His permission? He knows what is [presently] before them and what will be after them, and they encompass not a thing of His knowledge except for what He wills. His Kursi extends over the heavens and the earth, and their preservation tires Him not. And He is the Most High, the Most Great.”-(The Quran 2:255).

Christianity/Judaism (The Bible):

‘Jesus answered, “The most important commandment is this: ‘Listen, O Israel! The Lord our God is the one and only Lord.”-(The Bible, Mark 12:29)

Hinduism (Chandogya Upanishad):

“He is One only without a second.”-(Chandogya Upanishad 6:2:1).

The main or core messages of these religions are the same and one. That is, there is only one God. And the only one God is Allah (the almighty).