Ibn Ajiba's The Book of Ascension to the Essential Truths of Sufism
Certainty (yaqin) is the inner peace we find in God, arising from a knowledge that is unchanging, inalterable, and unaffected by transient events; it is the removal of doubts by a consciousness of the unseen.
The signs [of certainty in those who possess it] are three: they look beyond human beings when they are in need, they do not laud those who give to them, and they do not blame those who do not.
Certainty for the generality of believers rests in realizing the oneness of the divine acts so that they are at peace with God whether He gives or withholds. For the elect, it rests in realizing the oneness of the divine qualities such that they see creatures themselves as lifeless, without movement or rest except by God. For the elect of the elect, it is in realizing the oneness of God’s essence so that they witness Him by all things and in all things, and do not witness anything else besides Him.
Yaqin is derived from the root y-q-n, “to know something with certainty.” Forms of this root appear approximately 30 times throughout the Quran, as in the oft-repeated phrase, qawmun yuqinun: “a folk who are certain (in their faith).”
This note is expanded further in The knowledge of certainty, the eye of certainty, and the truth of certainty (ilm al-yaqin, ayn al-yaqin, wa haqq al-yaqin)