I think many times people have a blindspot in life, especially when it comes to spiritual things. This painting is a reminder that there are forces lurking around us ("our struggle is not against flesh and blood..."). For that very reason, we need to be prepared for battle and put on the full armor of God. The Red Arremer that is lurking behind Arthur represents the spiritual forces of evil. Arthur's shield bears the image of the 1up mushroom and the invincibility star to represent eternal life.  The background was inspired by a Frank Frazetta painting. 

"Finally, be strong in the Lord and in his mighty power. Put on the full armor of God, so that you can take your stand against the devil’s schemes. For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms. Therefore put on the full armor of God, so that when the day of evil comes, you may be able to stand your ground, and after you have done everything, to stand. Stand firm then, with the belt of truth buckled around your waist, with the breastplate of righteousness in place, and with your feet fitted with the readiness that comes from the gospel of peace. In addition to all this, take up the shield of faith, with which you can extinguish all the flaming arrows of the evil one. Take the helmet of salvation and the sword of the Spirit,which is the word of God." Ephesians 6:10-17