Who was Shri Guru Nanak Dev Ji ? what was his mission?

Guru Nanak’s message is that of simplicity and the subjective spiritual connection with the supreme energy. He emphasized upon reasoning and the need for the mankind to find meaning. Without meaning nothing holds. That is why he challenged the rituals.

He did not debunk them outright. He asked, he inquired, he gave the defenders of the rituals the chance to justify the logic to him. When they failed to do so, he propounded his simple three fold principle—Kirat Karo, naam japo, vand Chhako. Do you job, meditate and share your wealth. 

Yet another important aspect of his message is that of unconditional inclusiveness. He never asked anyone to convert or believe in this god or that god. He included multiple streams of faith in his talks and paid respect to them. If he disagreed, he disagreed respectfully. He did not disagree on the basis of faith. He disagreed on the basis of reason. He would never approve of any ritual that defied logic and sustained itself on the tradition and superstition. And also, he was a strong proponent of Karma, as in doing your job properly and honestly. He said that you can find god in your work if you do it with meditative concentration. There is no need to go and seek him in the woods or at places of worship.