Do you know the right that Allah has over you?.

 Allah Almighty has many rights over His servants, but there is a basic right that distinguishes the believers from the disbelievers and the polytheists, and the right that Allah has over His servants is that they should worship Him alone, not associating anything with Him, and Allah does not forgive that to be associated with Him, but He forgives what is less than that for whom He wills as He has stated in Ayat (48,116) of Surat Alnisa', and Allah assures in these two verses respectively that whoever associates with Allah, then indeed, he has fabricated a clear sin, and that whoever associates with Allah, then indeed, he has gone astray into a far error, and Allah has declared that He has forbidden Paradise for those who associate partners with Him as He has stated in Aya (72) of Surat Alma'idah, and as narrated by Jabir bin Abdullah: "A man came to the Messenger of Allah and said Messenger of Allah! What are the two things quite unavoidable?, He replied whoever dies without associating anything with Allah would enter Paradise, and whoever dies associating anything with Allah would enter the Fire" (Sahih Muslim 93 a).

  And as narrated by Mu'adh bin Jabal: "While I was riding behind the Prophet (PBUH) and between me and Him there was only the back of the saddle, He said O Mu'adh! I replied Labaik: at your service, O Messenger of Allah and Sa'daik: at Your pleasure, then He proceeded for a while, then He said O Mu'adh! I replied Labaik, O Messenger of Allah and Sa'daik, then He proceeded for a while, then He said O Mu'adh! I replied Labaik, O Messenger of Allah and Sa'daik, He said Do you know what is Allah's right over His servants?, I said Allah and His Messenger know best, He said Allah's right upon His servants is that they should worship Him alone and not worship anything else besides Him, then He proceeded for a while and then said O Mu'adh bin Jabal! I replied  Labaik, O Messenger of Allah and Sa'daik, He said Do you know what is the right of the servants upon Allah if they do so?, I replied, Allah and His Messenger know best, He said the right of the servants upon Allah is that He will not punish them" (Sahih Albukhari 5967). 

 So associating partners with Allah is the greatest sin in the sight of  Allah, and He forgives what is less than polytheism for whom He wills, and as narrated by Abu Hurayrah: "I heard the Messenger of Allah (PBUH) saying there were two men among the Children of Israel, who were striving for the same goal, one of them would commit sin and the other would strive to do his best in worship, the man who exerted himself in worship continued to see the other in sin, he would say Refrain from it, one day, he found him in sin and said to him Refrain from it, he said Leave me alone with my Lord, Have you been sent as watchful over me?, he said I swear by Allah, Allah will not forgive you, nor will He admit you into Paradise, their souls were taken back, and they met together with the Lord of the worlds, He said to this man who had striven hard in worship Had you knowledge about Me or Had you power over that which I had in My Hand?, and He said to the man who sinned Go and Enter Paradise by My mercy, and said about the other Take him to the Fire, Abu Hurayrah said By the One in whose Hand my soul is, he spoke a word by which this world and the Hereafter of his were destroyed" (Sunan Abu Dawood 4901).