The word(s) "unbelief" appears 17 time(s) in 16 verse(s) in Quran in Abdullah Yusuf Ali translation.

(1) Would ye question your Messenger as Moses was questioned of old? but whoever changeth from Faith to unbelief, Hath strayed without doubt from the even way.

(2) They ask thee concerning fighting in the Prohibited Month. Say: "Fighting therein is a grave (offence); but graver is it in the sight of Allah to prevent access to the path of Allah, to deny Him, to prevent access to the Sacred Mosque, and drive out its members." Tumult and oppression are worse than slaughter. Nor will they cease fighting you until they turn you back from your faith if they can. And if any of you Turn back from their faith and die in unbelief, their works will bear no fruit in this life and in the Hereafter; they will be companions of the Fire and will abide therein.

(3) When Jesus found unbelief on their part He said: "Who will be My helpers to (the work of) Allah?" Said the disciples: "We are Allah's helpers: We believe in Allah, and do thou bear witness that we are Muslims.

(4) Nor would he instruct you to take angels and prophets for Lords and patrons. What! would he bid you to unbelief after ye have bowed your will (To Allah in Islam)?

(5) And the Hypocrites also. These were told: "Come, fight in the way of Allah, or (at least) drive (The foe from your city)." They said: "Had we known how to fight, we should certainly have followed you." They were that day nearer to unbelief than to Faith, saying with their lips what was not in their hearts but Allah hath full knowledge of all they conceal.

(6) Let not those grieve thee who rush headlong into unbelief: Not the least harm will they do to Allah: Allah's plan is that He will give them no portion in the Hereafter, but a severe punishment.

(7) Those who purchase unbelief at the price of faith,- not the least harm will they do to Allah, but they will have a grievous punishment.

(8) Of the Jews there are those who displace words from their (right) places, and say: "We hear and we disobey"; and "Hear what is not Heard"; and "Ra'ina"; with a twist of their tongues and a slander to Faith. If only they had said: "What hear and we obey"; and "Do hear"; and "Do look at us"; it would have been better for them, and more proper; but Allah hath cursed them for their unbelief; and but few of them will believe.

(9) Those who believe, then reject faith, then believe (again) and (again) reject faith, and go on increasing in unbelief,- Allah will not forgive them nor guide them nor guide them on the way.

(10) O Messenger! let not those grieve thee, who race each other into unbelief: (whether it be) among those who say "We believe" with their lips but whose hearts have no faith; or it be among the Jews,- men who will listen to any lie,- will listen even to others who have never so much as come to thee. They change the words from their (right) times and places: they say, "If ye are given this, take it, but if not, beware!" If any one's trial is intended by Allah, thou hast no authority in the least for him against Allah. For such - it is not Allah's will to purify their hearts. For them there is disgrace in this world, and in the Hereafter a heavy punishment.