Why does God allow evil to enter this world? Because God knows he could afford it. God knows he could pay the cost of reversing all the works of evil with the blood of Jesus, His son.
“It was the last and most important day of the Feast. Jesus stood up and spoke in a loud voice. He said, "Let anyone who is thirsty come to me and drink. Does anyone believe in me? Then, just as Scripture says, streams of living water will flow from inside him." When he said this, he meant the Holy Spirit. Those who believed in Jesus would receive the Spirit later. Up to that time, the Spirit had not been given. This was because Jesus had not yet received glory.” (John 7:37-39)
Jesus is the well of the living water, the Holy Spirit. 