The path to knowing monotheism in belief, behavior, and action

 How can we know the truth of monotheism in belief, behavior, and action?

The path, thanks to Allah, is made easy. 

A believer should hold themselves accountable, adhere to the truth, and be influenced by those who apply the texts to themselves. This will help them remain steadfast in the oneness of Allah and sincerity towards Him, consistently acting upon it and inviting others to do the same. This practice will become second nature, and they will not be swayed by those who try to hinder or confuse them.

The key is to focus on this matter, hold oneself accountable, and understand it well to avoid confusion and falling for doubts.