… If any man thinks that he  knows  any thing, he  knows  nothing yet as he  ought  to know.  But if any man loves God, the same is known of Him (1 Corinthians 8:2-3)
 

Muslims "know" that Mohammed is God's prophet, and the Koran is the Word of God.

Latter-Day Saints "know" that Joseph Smith is God's prophet, and the  Book of Mormon is the Word of God.

Born-again Christians "know that they know that they know" that they are saved through faith Jesus Christ, and there is no other way to heaven but through Him.

Ecumenists notwithstanding, it is quite clear that not all of those who "know" can be correct.  Being certain doesn't mean being right.

"Certainty" is  not strong faith.  On the contrary, "certainty" is usually a  cover for weak faith.  "Certainty" means that there are assumptions that one never examines, there are dogmas which one never questions.  "Certainty" means that I shelter my faith, not that my faith shelters me.

"Certainty" paves the way for fanaticism.  "Certainty" means there can be no discussion or dialog with those who disagree.  "Certainty" means I refuse to make myself vulnerable, to expose myself open myself to the ways others think and feel.

True faith in Jesus Christ is not an intellectual or emotional "certainty", but an absolute dependence upon Him in all things, whether practical or spiritual, physical, emotional, or mental. It is through this utter dependence, through seeking His will and His face in all things no matter how trivial, that He reveals His love, His care, His knowledge of the innermost recesses of my heart and the deepest needs of my soul.

"Depending upon" is not the same as "certainty".  "Depending upon" means being honest with myself and with God about the second thoughts, doubts, anxieties, and fears which rise up within me.  "Depending on" means  that when doubts and fears do arise, then I run to the One I depend on, seeking support, reassurance, and reaffirmation.  "Depending upon"  means being willing to lay my faith on the line, as Elijah laid his faith on the line in his contest with the prophets of Baal (1 Kings 18).

Prayer:  Lord, thank You that You require only a mustard seed of faith.  Help us plant the seed in good ground, that our roots may sink deep into You (Isaiah 27:6).  When our faith falters, Lord, though you may rebuke us  (Luke 8:25), please do not fail to strengthen us.