Ar ridda (apostasy) is: disbelief after Islaam.
It can be through statement, action,i’tiqaad (belief) and doubt.
So whoever: 1) makes shirk with Allaah or 2) rejects His ruboobeyah (Lordship) or His wahdaaniyah (Oneness) or 3) (rejects) an Attribute from amongst His Attributes or 4) (rejects) some of His Books or His Messengers or 5) insults/reviles Allaah or His Messenger (sal Allaahu alaiyhi wa sallam) or 6) (rejects) anything from those forbidden matters, whose forbidden nature is agreed upon or who seeks to make (that thing) permissible or 7) rejects the obligation of a pillar from amongst the five pillars of Islaam or has a doubt about the obligation of this (pillar) or 8) (who doubts) the truthfulness of Muhammad (sal Allaahu alaiyhi wa sallam) or any of the other Prophets or 9) (who doubts) the Resurrection after Death or 10) who prostrates to an idol or a star or anything like that -
then indeed he has committed kufr (disbelief) and apostatised from the religion of Islaam.
So it is upon you to read the chapters of the ruling of apostasy from the Books of Islaamic Fiqh – for indeed they (the scholars of fiqh) paid great attention to this – may Allaah have mercy upon them.
In this way you will know - from the preceding examples - about the apostasy through speech, action and belief and the manner of apostasy through doubt.