And Jesus said unto them,Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.

-Bible (NewTestament)
St Matthew17:20.

And straightway the father of the child cried out, and said with tears, Lord, I believe; help thou mine unbelief.

-Bible (NewTestament)
St Mark 9:24.

All we have gained then by our unbelief Is a life of doubt diversified by faith, For one of faith diversified by doubt: We called the chess-board white,öwe call it black.

-Browning, Robert
  Men and Women,'Bishop Blougram's  Apology'.

Blind unbelief is sure to err, And scan his work in vain; God is his own interpreter, And he will make it plain.

-Cowper,William
  Olney Hymns,'Light Shining out of Darkness'.

Belief consists in affirming the affirmations of the soul; unbelief, in denying them.

-Emerson, RalphWaldo
  Representative Men,'Montaigne; or, The Skeptic'.

I do not believe in Belief† Lord I disbelieveöhelp thou my unbelief.

-Forster, E(dward) M(organ)
Two Cheers for Democracy,'What I Believe'.

Thereisnostrengthinunbelief.Eventheunbeliefof what is false is no source of might. It is the truth shining from behind that gives the strength to disbelieve.

-MacDonald, George
The Marquis of Lossie (published1906).