LOOK up now, weak wretch, and see what thou art. What art thou, and what hast thou merited, thus to be called of our Lord? What weary wretched heart, and sleeping in sloth, is that, the which is not wakened with the draught of this love and the voice of this calling! Beware, thou wretch, in this while with thine enemy; and hold thee never the holier nor the better, for the worthiness of this calling and for the singular form of living that thou art in. But the more wretched and cursed, unless thou do that in thee is goodly, by grace and by counsel, to live after thy calling. And insomuch thou shouldest be more meek and loving to thy ghostly spouse, that He that is the Almighty God, King of Kings and Lord of Lords, would meek Him so low unto thee, and amongst all the flock of His sheep so graciously would choose thee to be one of His specials, and sithen set thee in the place of pasture, where thou mayest be fed with the sweetness of His love, in earnest of thine heritage the Kingdom of Heaven.
You can get the Cloud of Unknowing in a variety of translations and formats.