In a short statement, all major religions have a common. That is “Monotheism”.
According to the Oxford dictionary, the definition of monotheism is-
“The doctrine or belief that there is only one God.”
Although some Christians and Hindu(s) don’t believe in monotheism. But it is a fundamental in Hindu and Christian scriptures. And In Islam, monotheism is the most important and first part to believe and understand. No Muslim can be a Muslim without understanding and believing it. In Muslim culture and society, monotheism is widely believed and observed.
In the Bible, Jesus (peace be upon him) said:
“The most important one,” answered Jesus, “is this: ‘Hear, O Israel: The Lord our God, the Lord is one.”-(Mark 12:29)
In the Quran, Allah (the almighty) said:
“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.”-(Quran 2:255)
Vedas and Bhagavad-Gita are considered as most sacred scriptures in Hinduism. Following are a few lines where they talk about monotheism and concept of God:
“He is one only without a second”-(Chandogya Upanishad 6:2:1)
“Of him, there are neither parents nor lord.”-(Svetasvatara Upanishad 6:9)