In plain words
The ignorant, the faithless, and the doubting person perish. For one given to doubt there is neither this world, nor the next, nor happiness.
Word for word
The ignorant and the faithless and the doubter perishes; neither this world is, nor the next, nor happiness, for the doubter.
The commentators
Bhagavadgītābhāṣyam
अत्र संशयः न कर्तव्यः, पापिष्ठो हि संशयः ; कथम् इति उच्यते —
अज्ञश्च अनात्मज्ञश्च अश्रद्दधानश्च गुरुवाक्यशास्त्रेषु अविश्वासवांश्च संशयात्मा च संशयचित्तश्च विनश्यति । अज्ञाश्रद्दधानौ यद्यपि विनश्यतः, न तथा यथा संशयात्मा । संशयात्मा तु पापिष्ठः सर्वेषाम् । कथम् ? नायं साधारणोऽपि लोकोऽस्ति । तथा न परः लोकः । न सुखम् , तत्रापि संशयोत्पत्तेः संशयात्मनः संशयचित्तस्य । तस्मात् संशयो न कर्तव्यः ॥ ४० ॥
In plain words
Don't let doubt in here. Doubt is the most ruinous thing there is. How so? That is what is said next —
'The ignorant one' — one who doesn't know the Self — and 'the faithless one', distrustful of a teacher's word and of scripture, and 'one of doubting nature', whose mind is riddled with doubt — all of these perish. The ignorant and the faithless do perish, yes, but not the way the doubter perishes. The doubter is, in fact, the worst off of all. Why? Because for him, not even this ordinary world really exists, let alone any other world beyond it. And there's no happiness for him either — since doubt creeps in there too, for someone of a doubting nature, a doubting mind. That is why doubt should never be indulged.
Close to the Sanskrit
Here doubt is not to be entertained; for doubt is most sinful. "How?" — thus it is said —
'The ignorant one' and one who does not know the Self, and 'the faithless one', distrustful regarding the teacher's word and the scriptures, and 'one of doubting nature', one of doubting mind, perishes. The ignorant one and the faithless one, although they both perish, do not perish the way the doubter does. The doubter, however, is the worst of all. How so? For him, not even this ordinary world exists. Likewise there is no other world either. Nor is there happiness — since doubt arises even there, for one of doubting nature, of doubting mind. Therefore doubt should never be entertained.