Religious, but Unrighteous: Making a mess of religion in Nigeria

According to Quran 2:139, Prophet Mohammed (pbuh) instructed his followers to ask the people of the scripture (Jews and Christians); " [why] do you argue with us about Allah while He is our Lord and your Lord? Ours are our works and yours your works. We look to Him alone”. This is the clearest acceptance by Prophet Mohammed (pbuh) that Jews, Christians and Muslims worship the same God. If that is the case, why are they fighting each other?