While looking for some material about the oneness of Allah as to why we need to believe in one God alone, I came across the following article well written by a Muslim scientist. Please share it with your non-Muslim friends if possible.
In fact, this is THE most important topic you could ever have in your lifetime as it is going to affect your life in this world and the world hereafter. Doing shirk, making partners with Allah is, in fact, the biggest sin in the eyes of Allah and we need to spread the word about this most hineous crime to fellow brethren who are not yet Muslims. We need to love all of humanity no matter which creed they belong to, where they come from and what their views are about religion and oneness of Allah, the Almighty.