What happens to a Yogi who does not attain to the highest condition in one birth? We have the words of Sri Krshna in the Gita resolving this query from Arjuna.
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अयतिः श्रद्धयोपेतो योगाच्चलितमानसः ।
अप्राप्य योगसंसिद्धिं कां गतिं कृष्ण गच्छति ॥
Arjuna said: He who takes up Yoga with faith, but cannot control himself with the mind wandering away from Yoga, failing to attain perfection in Yoga, what is his end, O Krishna?
कच्चिन्नोभयविभ्रष्टश्छिन्नाभ्रमिव नश्यति ।
अप्रतिष्ठो महाबाहो विमूढो ब्रह्मणः पथि ॥
Does he not, O mighty-armed, lose both this life (of human activity and thought and emotion which it has left behind) and the Brahmic consciousness to which it aspires and failing from both perish like a dissolving cloud?
पार्थ नैवेह नामुत्र विनाशस्तस्य विद्यते ।
न हि कल्याणकृत्कश्चिद् दुर्गतिं तात गच्छति ॥
The Blessed Lord said: O son of Pritha, neither in this life nor hereafter is there destruction for him; never does anyone who practises good, O beloved, come to woe.
प्राप्य पुण्यकृतां लोकानुषित्वा शाश्वतीः समाः ।
शुचीनां श्रीमतां गेहे योगभ्रष्टोऽभिजायते ॥
Having attained to the world of the righteous and having dwelt there for immemorial years, he who fell from Yoga is again born in the house of righteous people, or into a family of rich aristocracy.
अथवा योगिनामेव कुले भवति धीमताम् ।
एतद्धि दुर्लभतरं लोके जन्म यदीदृशम् ॥
Or he may be born in the family of the wise Yogin; indeed such a birth is rare to obtain in this world.
तत्र तं बुद्धिसंयोगं लभते पौर्वदेहिकम् ।
यतते च ततो भूयः संसिद्धौ कुरुनन्दन ॥
There he recovers the mental state of union (with the Divine) which he had formed in his previous life: and with this he again endeavours for perfection, O joy of the Kurus.
पूर्वाभ्यासेन तेनैव ह्रियते ह्यवशोऽपि सः ।
जिज्ञासुरपि योगस्य शब्दब्रह्मातिवर्तते ॥
By virtue of his former practice [ in previous lives] he is irresistibly carried on. Such a seeker after the knowledge of Yoga goes beyond the range of śabdabrahmā – the scriptures.
प्रयत्नाद्यतमानस्तु योगी संशुद्धकिल्बिषः ।
अनेकजन्मसंसिद्धस्ततो याति परां गतिम् ॥
The Yogin, endeavouring persistently, purified from sin, perfecting himself through many lives attains to the highest goal.
तपस्विभ्योऽधिको योगी ज्ञानिभ्योऽपि मतोऽधिकः ।
कर्मिभ्यश्चाधिको योगी तस्माद्योगी भवार्जुन ॥
The Yogin therefore is greater than the ascetic, greater than the men of knowledge, greater even than the men of works; become then the Yogin, O Arjuna.