Aslan is the Christ-figure for Narnia, as Lewis himself explained in a letter he wrote in 1961:
The whole Narnian story is about Christ...Supposing there really was a world like Narnia...and supposing Christ wanted to go into that world to save it (as he did ours), what might have happened? The stories are my answer. Since Narnia is a world of talking beasts, I thought he would become a talking beast there as he became a man here. I pictured him becoming a lion there because a) the lion is supposed to be the king of beasts; b) Christ is called 'the Lion of Judah' in the Bible.
I started wondering, what if C.S. Lewis had chosen instead to make Aslan a very ordinary animal. A dog, say -- not pure-bred. Just a mutt. A lab-pitbull mix, is what I was thinking. How would that have changed things?
Aslan is a wonderful character, but you have to admit, he looks kingly and noble from the get-go, all the more so when you're voiced by Liam Neeson. I wonder if one of the reasons -- one of the many reasons -- Jesus stirred up such violent passions against him is that he didn't look the part. He didn't look like a Lion of Judah; he looked like your average, skinny street dog of Nazareth.
But even though he didn't look the part, he still sounded the part. If you look at the two things Jesus says while on the cross, they sound mighty kingly, almost noblesse oblige: "Father, forgive them, for they do not know what they are doing," and "Today you will be with me in paradise." There's even some arrogance in that. Here he is, powerless and nailed down, and he still speaks as though it is in his power to dispense forgiveness and welcome.
Both in Narnia and in our world, people felt the need to cut the Christ down to size. In Narnia, is was because they did see his kingship; in our world, because they didn't see it at all.