The worst major sin is Shirk. It is of two kinds:

a) To hold that Allah has an equal, whereas He has created you, and to worship another with Him, whether it be a stone, tree, sun, moon, prophet, sheikh, star, angel or other. This is the greatest shirk mentioned by Allah, the Most High, the Almighty, says, {Allah forgiveth not (the sin of) joining other gods with Him; but He forgiveth whom He pleaseth other sins than this; one who joins other gods with Allah hath strayed far, far away (from the right).} (An-Nisa:l16)

{False worship is indeed the highest wrong-doing.} (Luqman:13)

{Whoever joins other gods with Allah; Allah will forbid him the Garden, and the Fire will be his abode.} (Al-Ma'idah:72)

The Qur'anic verses concerning this are very numerous. It is being absolutely certain that whoever ascribes associates to Allah and dies in such a state is one of Hell's inhabitants, just as whoever believes in Allah and dies as a believer is one of the inhabitants of Paradise, even if be should be punished first.

The Prophet (pbuh) said, "Shall I tell you of the worst major sins? Worshipping others with Allah, showing disrespect to parents, giving a false statement, and testifying to the truth of a falsehood. " And he kept repeating it until we were telling ourselves (out of sympathy for him because of the strain of repeating it), "If only he would be silent.”1

"Avoid the seven heinous sins.”2

and he mentioned ascribing associates to Allah.

"Kill whoever converted his religion (Islam).” 3

Allah, the Most High, says, {Whoever expects to meet his Lord, let him work righteous, and, in the worship of his Lord, admit no one as partner.} (AL-Kahf:llO)

i.e., does not show off in good works "Beware of the lesser shirk." They asked, "What is the lesser shirk?" He said, "Showing off in good works.

Allah, the Almighty will say when recompensing the mankind for their deeds: Go to those whom you showed off to in the world to see how they could recompense you. .” 1

The Prophet (pbuh) said, "If anybody associates something with Me, I am not affected. If anyone associates somebody with Me, in any way, I reject him and his act of associating.”2

The Prophet (pbuh) said, "A person who does some good work in order to boast will have his faults exposed by Allah Most High and a person whose motive in doing some good deed is to show it, Allah the Most High will treat him as a hypocrite.”3

The Prophet (pbuh) said, "There may be a faster who does not gain from his fasting but hunger and thirst.” A wise man said, "Just as the one who shows off in good works is the one who fills his purse with pebbles and leaves for the market to buy food. If he opened it before the seller he will be stoned with it in his face. He gains nothing but the people's inquiry about what fills his purse, nothing more. Thus, the one who shows off in good works will have no value but the people's saying that he did good and he will not be rewarded in the Hereafter." Allah, the Almighty, says, {And We shall turn to whatever deeds they did (in this life), and We shall make such deed as floating dust scattered about.} (AI-Furqan:23)

The Messenger of Allah (pbuh) said, "On the Day of Judgment some groups of people will be brought near Paradise. Having smelt its fragrance, and seen palaces and blessings therein, they will hear a call saying "Take them away, it is not for them." Accordingly, they return so sorrowful that no one could have such feeling. They will call upon Allah, "0 Lord were we entered into the Hell-Fire before seeing what You have prepared for Your Friends, it would have been better." Then Allah will say, " That is what I want. When you were alone you defied Me with grievous sins, but when you meet people you showed off in good works, You only respected people not Me, You only refrain (from things) for their sake. Hence, today, I will severely torment you in addition to depriving you from My bountiful reward.” 4

It was related that a man asked the Messenger of Allah (pbuh)

"How can I be redeemed?" The Prophet said, "If you do not deceive Allah" The man asked, "How is Allah deceived?" He replied, "When you do something which Allah and His Messenger commanded you to do but you intended other than pleasing Allah. Beware of ostentation, it is the lesser shirk (polytheism); On the Day of Judgment, the ostentatious will be called at the presence of all creatures by four names: O Ostentatious, O traitors, O corrupt, O loser; what you did is wasted and your reward is nullified. We will not reward you, go and get your reward from whomever you acted to deceive?” 1

A wise man was asked, "Who is the sincere one?" He replied, "It is he who hides his good deeds as he does with his bad deeds." Someone asked, "What is the good of sincerity?" He said, "When you dislike to be praised by people." Al-Fudail Ibn Eiad, may Allah be pleased with him said, "To refrain from an action for the sake of people is ostentation. To do something for their sake is shirk (polytheism). But sincerity is to be aware of them.

O Allah! Let us be away from them and forgive us.