How can man hope to attain the purpose of his life, or obtain the Mercy of Allah for the sins of his life when he does not serve Allah exclusively? This person has no true understanding of his own true position before Allah and his role in this life; nor of the working of the Divine Will and Purpose.
ذَٲلِكُم بِأَنَّهُ ۥۤ إِذَا دُعِىَ ٱللَّهُ وَحۡدَهُ ۥ ڪَفَرۡتُمۡۖ وَإِن يُشۡرَكۡ بِهِۦ تُؤۡمِنُواْۚ فَٱلۡحُكۡمُ لِلَّهِ ٱلۡعَلِىِّ ٱلۡكَبِيرِ
(The answer will be:) “This is because when Allah was invoked as the Only (object of worship), you did reject Faith, but when partners were joined to Him, you believed! The command is with Allah, Most High, Most Great!” Quran 40:13