Assalam o alaykum,
Below is an excerpt from Imam Sayyid Anwar Shah Kashmiri's (may Allah have mercy on him) magnum opus, Ikfar al-Mulhidin, where he lists those essential matters which are so delicate that Muslims can't be declared kafir over these. These few issues are such that even great scholars have slipped in these matters let alone laymen. There is a need for great caution when declaring Muslims as kafir over these few points listed below.
Those Essential Matters which if denied will NOT Entail Kufr
"But, those essential matters and correct beliefs which are too subtle and thorny to be understood by common human mind like the issue of taqdir (destiny),  the reality and exact condition of the punishment in the grave, the issue of istiwa on the Throne,  the reality and exact condition of Allah’s descent to the sky of the world in the last part of night, other similar mutashabih (ambiguous) issues,  the reality and exact condition of Allah’s Essence and His Divine Attributes, etc, if such matters reach to an extent of tawatur [mass-transmitted], so one who will deny these issues categorically in spite of their knowledge, we shall declare him kafir without any hesitation. But if he does not deny [any of] these altogether, rather deviates while discussing and exploring its nature and condition, and fixes a point by himself and claims it to be only true whereas it is baseless according to the people of truth e.g. he regards the punishment of grave as spiritual punishment, or he explains the istiwa on the Throne as sitting on the Throne and says that Allah Most High sits on the Throne, then we shall consider such Muslim excused and will attribute his deviation to ignorance, but will not regard him as kafir.
For the abovementioned research and detail, you should consult the treatise of 'Allamah Ibn Rushd al-Hafid (may Allah have mercy on him) named 'Fasl al-Maqal wa al-Kashf ‘an Manahij al-Adillah'. He has logically proved that such a person is surely deviant and ignorant, but not kafir."