It is often argued that bhakti-yoga is not the only way to attain samādhi (trance) because Patañjali describes various other methods in his Yoga Sūtra.
But is that argument really valid, or is it a misinterpretation of Patañjali's system? Although Patañjali suggests multiple ways to attain samādhi, one should note that there are seven types of samādhi in the Yoga Sūtra of Patañjali.
vitarka-vicārānandāsmitārūpānugamāt samprajñātaḥ ||17||
Six types comprise samprajñāta-samādhi, followed by one type of asamprajñāta-samādhi.
1) savitarka-samprajñāta-samādhi.
2) nirvitarka-samprajñāta-samādhi.
3) savicāra-samprajñāta-samādhi.
4) nirvicāra-samprajñāta-samādhi.
5) sānanda-samprajñāta-samādhi.
6) sāsmitā-samprajñāta-samādhi.
7) asamprajñāta-samādhi.
The six types of samprajñāta-samādhi are NOT the perfection of samādhi. They are the gradual stages leading towards the final goal. Just as an archer practices by shooting at progressively smaller targets, each stage of samādhi has progressively subtle targets of concentration.
The final perfection of samādhi is only attained by surrender to Īśvara. That is clearly stated in Yoga Sūtra 2.45:
samādhi-siddhir īśvara-praṇidhānāt — the perfection of samādhi comes through surrender to Īśvara (Yoga Sūtra 2.45).
In context, īśvara-praṇidhāna is the fifth of the five niyamas (2.32) and the third component of kriyāyogaḥ (2.1) Therefore, īśvara-praṇidhāna is not simply one optional method among many. In Patañjali's system, it is absolutely mandatory just to begin the process, what to speak of attaining the ultimate perfection.
The commentary of Vyāsa, which is traditionally considered inseparable from the original sūtras, defines īśvara-praṇidhāna as viśeṣa-bhakti: "intense devotion to the Supreme Lord".
Furthermore, the fourth niyama, svādhyāya, culminates in meeting with the particular form of the Supreme Lord who is the object of one's devotion.
svādhyāyād iṣṭadevatāsamprayogaḥ ||44||
"From the study of scriptures and japa there is meeting with the particular form of the Supreme Lord who is the object of one's devotion."||2.44||
In conclusion, surrender and devotion to īśvara is compulsory within the Yoga Sūtra of Patañjali. Of course, everyone is free to suggest other means to attain perfection, but they cannot do so coherently on the authority of the Yoga Sūtra of Patañjali.