Because spiritually blind people do not understand the merits of Christian faith and practice.
Now, in order to qualify that you understand the Christian faith, I'd really like to know if you meet the spiritual qualification of being alive in Christ.
This is a classic theocratic argument. It is the antithesis of America's foundation on freedom of conscience. You can't claim to have spiritual superiority without explaining why rationally. That's what religious tyrants have always tried to do. King Henry the 8th burned more than a hundred Catholic monasteries and abbeys while spurring his troops on with the promise of superior spirituality.