Shaikh Ibn Bāz, may Allāh have mercy on him, said:
Al-Yaqīn [certainty] means to be a believer in Allāh upon definite resolve and certainty; one believes that Allāh is his Lord, the one, True One, to be worshiped by him; that no one but He deserves to be worshiped; that He is the creator of all things; that He is perfect in Himself, His names, attributes, and actions; that He, be He glorified for His perfection, must be worshiped alone and singled out for worship.
And a believer must be on guard against the evils of his [own] tongue, in [ever] saying things that diminish his Lord, deny His attributes, or make light of what He has commanded him with.
A believer knows with certainty that He, be He glorified for His perfection, is the only One who deserves to be worshiped; that He is the Lord of all creation; that He is Al-Khallāq [the Ever-Creating], Al-ʿAlīm [the All-Knowing]; that He is the only One who is deserving of worship; that there is no lord besides Him nor any creator besides Him; that no one besides Him deserves to be worshiped, far above all is He in His perfection and All-High.
And he knows with certainty that He will gather mankind together on the Day of Resurrection; He will pay them back [all] for their deeds—with good, if good, and with bad, if bad.
He knows with certainty that Allāh, be He glorified in His perfection, will fulfill His promises, so He will put the believers in Paradise as He has promised them, and He will put the disbelievers in the Fire, as He has promised them, far above all is He in His perfection and All-High.
And in this manner, everything that Allāh and His Messenger has informed him about in the Qurʾān or in authentic ḥadīths, a believer believes in—he believes in that and has no doubts about that.