The main problem with these types of claims, as Dr. William Lane Craig recently pointed out in his dialogues with Dr. Lawrence Krauss in Australia, is that they are 100 years out of date! But they are alive and well on the internet.
In this featured article, Christian case maker J. Warner Wallace demonstrates that Jesus is not a retelling of the Horus myth. He also offers some excellent resources to explore this topic more in-depth.
You can check it out here.
Ah, but what about Mithras? See here.
Courage and Godspeed,