Bhagavad Gita · Jnana Karma Sannyasa YogaChapter 4, Shloka 40 (4.40)

Three states — one the hardest. Ajña (ignorance), aśraddadhāna (lack of faith), saṃśayātmā (consumed by doubt). All three fall — but the doubter most of all.

Shankaracharya: ajñaś ca anātmajñaś ca, aśraddadhānaś ca guru-vākya-śāstreṣu aviśvāsavān — one who does not know the self, and one who has no trust in the words of the guru and in scripture. Ajñāśraddadhānau yadyapi vinaśyataḥ na tathā yathā saṃśayātmā; saṃśayātmā tu pāpiṣṭhaḥ sarveṣām — the ignorant and the faithless fall too, but not as much as the doubter; he is the worst of all. Why? Nāyaṃ sādhāraṇo 'pi loko 'sti … na sukham, tatrāpi saṃśayotpatteḥ — for him there is not even this world common to all, nor the next, nor happiness — because doubt arises everywhere.

Acharya Ramanuja: ajñaḥ upadeśa-labdha-jñāna-rahitaḥ (the ignorant: one without the knowledge gained through teaching); aśraddadhānaḥ atvaramāṇaḥ — the faithless, meaning one who shows no eagerness (enthusiasm) to grow in knowledge; upadiṣṭe ca jñāne saṃśayātmā — one who doubts the knowledge taught. For him dharmārtha-kāmādi-puruṣārthāḥ ca na siddhyanti, kuto mokṣaḥ — even dharma, artha and kama are not achieved, so how could liberation be? For all human goals are achieved through scriptural action, and that action only dehātirikta-ātma-niścaya-pūrvaka — with the conviction of a self distinct from the body.