Both the atheist and the Christian will agree on this: we came from dust and to dust we shall return. Apart from God we have no hope, for we are dust and ashes.
The ashes on our forehead are not a beauty mark. They point to the ugliness of the sins within us. Yet they are also an appeal to God that he would wash us clean.
Fire, like sin, consumes all it touches. Our ashes are a sign of all we have lost or harmed by sin. However, Old Testament sacrifices were also offered up to God by fire. Our ashes are also a sign of our desire to sacrifice ourselves entirely to him.
We receive ashes on our forehead in the shape of a cross; ashes because we are sinners, and a cross because we are Christ’s.