Relatively speaking Sikhism is an especially young religion, dating back only to the 15th century; it’s much younger than its more widespread counterpart Christianity, for example.
It was started by Guru Nanak Dev who was the primary and arguably most well-known member of the Ten Gurus of Sikhism. He started the religion as a direct retort to his, and a small but fast-growing faction of like-minded individuals, dissatisfaction with the state and practice of 15th century Hinduism.
Guru Nanak Dev created a religion that cherry-picked from both Hinduism and Islam and introduced the concept of one deity, one God, and pushed forward many of the concepts of Hinduism that Guru Nanak Dev and the Ten Gurus of Sikhism found backward and outdated. The core values of Hinduism were quickly cemented and the religion fast gained followers, taking those followers and teachings from both the Hindu and Islamic religions and scriptures, respectively.
The Concept of One God
The concept of one true God is a core principle of Sikhism. The religion believes in one deity. He is all-knowing, eternal. He created the sun, the moon, the rain and the universe. He is both judge and jury and has the power to create and indeed destroy all. He was never born, he has and always will exist. He is neither good nor bad. He just is and always will be.
This concept of the one true God, as they believe, applies only to God, it does not apply to human beings who, in the teachings of Guru Nanak Dev, and Sikhism, are born inherently good. A contradiction of sorts, but an important distinction between the celestial being and his earth-born creations is made with such a statement.
The Worship of Idols
Followers of Sikhism give gifts and food to inanimate objects and idols in the belief that God resides in them and that praise, prayer, and worship of these objects will result is a direct intermediary to the celestial above. A vessel for direct communication with God, if you like. These objects are treated with the up-most respect and revelry and these practices inform the lives of millions of followers of Sikhism, day-in, day-out.
These actions and protocols are inherently polytheistic in origin, of course. Indeed these actions in Christianity would be considered blasphemous. A modern day example of idol worship in Sikhism is the custom of washing statues with milk, this is a ritual that continues year-in-year-out and is a fitting example of how Sikhs still hold on to these beliefs and teachings from Hinduism.
The core beliefs and principals of Sikhism most definitely define the religion as monotheistic, however, it’s not completely black and white as Sikhs still to this day celebrate many of the major events in the polytheistic calender, such as; Diwali, Hola Mahalla, and Vaisakhi and engage in the aforementioned idol worship. It is accurate to define the religion as monotheistic, but the religion has many, many distinct differences between the other two big monotheistic religions, Islam and Christianity. Enough differences that some find it hard to categorize it as one or the other.