Why did God allow the snake into the Garden of Eden?

God tests us everyday. The serpent was Adam and Eve's test to see if they were truly faithful to heir Lord. Instead Eve was tempted and proved to God how ignorant we humans are and how easily Satan can deceive us. Because of her sinful act, we have to follow God's rules in order to stay safe from Satan and prove to him we are worthy and loyal enough to fit into his kingdom. Now don't think that the sins of Eve doomed the entire human race, because it is in our blood and nature to sin. Everyone sins, and even if Eve didn't sin, someone was bound to do it.