Strongest Impediment in Sulook
Hakeemul Ummat Hazrat Mawlana Shah Ashraf Ali Thanawi ra said;
Although, all the sins and attachments to other-than Allah SWT are an obstacle in this path of sulook. Few indispensable ones are explained here.
First and foremost is opposition of the Sunnah of Prophet sallallaho alehey wasalam (in belief, acts of worship, especially ikhlaq/morals, mutual dealings, financial transactions and all other activities).
Taleem ud Deen, page 131, Taj Co. edition. (An essential reading in Nisab e Tasawwuf)
'abd | A. Reality of Tasawwuf, D. Acts of Worship