What would it take?
What would it take to convince you that all of this is real? That angels and demons are real? That God is real?
In Supernatural's third season Assault on Precinct 13 homage, "Jus in Bello", Agent Henriksen learns first-hand that demons exist. He's possessed during the siege and loses control of himself. When he's freed of demonic influence, his eyes are opened.
After the ordeal, everything he's seen and heard makes sense. The Winchester's weren't lying. All those crazy stories really happened. All the writings of their father are true. All of it's real - the book, the traditions, the legends, the artifacts, the people.
It is hard to believe. That is why faith is a gift of the Holy Spirit. Faith is a supernatural virtue that we cannot truly have without God's help.
To some, God gives words or visions or experiences that make it more clear. Even then, sin darkens our minds. We come up with every excuse that what we've seen or heard or felt has some other explanation. Anything - anything! - but certainty that God is real, because if God is real, then all of this counts. If God is real, then we can (and do) sin against Him. We can be apart from him in this life, and we can end apart from Him forever. Yet, Jesus teaches us that "blessed are those who have not seen and have believed." (John 20:29)
To all of us, God gives the gift of faith if we ask and dispose ourselves to receiving it. [162, 179] The gift remains with us as long as we remain disposed to it, free from sin.  We can fool ourselves. We can buy into the lies of the world, the flesh, or the devil. We can hide from that truth out of fear or false humility. But we need it. We need that truth, because He is the Truth, and we need Him. We can do nothing outside of Christ - the Way, the Truth, and the Life.