Question: Please clarify to us the difference between a believer and a Mushrik (one who associates others in worship with Allaah) keeping into account that there are some people who perform Salaah (prayer), Sawm (fasting), Hajj and all good deeds, but they make Tawassul (supplicating to Allaah through a means) by prophets and righteous people. Is this permissible or not?
Answer: If a Muslim practices an act of Kufr (disbelief), such as invoking the dead to cure the sick, enlarge the provision or grant offspring, they will be out of the pale of Islaam. All their deeds of Salaah, Sawm and the like will be in vain if they die on this status. But if a believer commits a misdeed that is not tantamount to Kufr, like robbery or dealing in Ribaa (usury), their affair will be left to Allaah, if He wills, He may forgive them and if He wills, He may punish them… read more here.