Tash and Aslan

In C.S. Lewis' book "The Last Battle", which is the last book in the Chronicles of Narnia series, the reader is introduced to a character named Tash. Tash is referred to as a malevolent deity or demon by those in Narnia. It can be reasoned that Tash is the direct opposite to Aslan in both appearance and action. Those who worship Tash are called the Calormenes. The Calormenes practice human sacrifice to appease Tash.

Latter on in the story, C.S. Lewis tells us that several of the Calormene do not believe in Tash. Several of these Calormene formulate a story that Aslan and Tash are the same and give him the name Tashlan. This confuses some of the Narnians which causes some Narnians to follow Tashlan.

Emeth, a Calormene soldier, does several things to serve Tash and when he searches to confront Tash latter on in the story, he goes searching into Aslan's Country. What he finds instead is Aslan himself. Aslan tells Emeth that everything that Emeth did in service to Tash that was good was done in service to Aslan. Aslan then explains that anything which is good is of Aslan and anything which is bad is not of Aslan. He explains that Emeth's pious devotion was really to Aslan, rather than to Tash, although Emeth had not been aware of this, and Emeth finds great happiness in this revelation.

What is C.S. Lewis saying in this story?