Answer
When Arjuna asked, the Lord discussed in the twelfth chapter of the Gita the distinction between the worship of the Nirguna-Nirakara God (Brahman) and His own Saguna-Sakara form (Narayana/Shri Krishna), and concluded that the yogis who worship the Saguna-Sakara God are the more excellent knowers of Yoga. One reason He gave for this is that for embodied beings, the worship of the formless is difficult and fraught with hardship, whereas the worship of the Saguna-Sakara is natural and joyful. The path of Bhakti is intrinsically connected with the worship of the Saguna-Sakara God (12:1–5).
In His essential nature, God is formless — imperceptible to sight, intangible, beyond dialogue, unmanifest, and without form. But for the governance of the manifest creation, the same Supreme Lord assumes a personal, embodied form as Narayana; and that very Narayana, when the need arises, sometimes incarnates in human form — as Shri Rama and Shri Krishna. Occasionally He also creates certain other avatars from His minor aspects, but for devotion-centered worship, these two are primarily regarded as the most suitable among His human incarnations.
In this essay, the discussion on Bhakti will be based principally on the Bhagavad Gita; however, the supplementary scriptures — the Bhagavata Purana and the Shri Ramcharitmanas — can lend greater clarity and deeper insight to the Gita's ideas. Therefore, we shall draw upon the thoughts of these scriptures wherever appropriate to corroborate and elucidate the Gita's teachings, as both of these, after the Gita, are among the greatest Vaishnava texts.
In ancient times, Prahlada and Dhruva; in the medieval period, the saint-poet Tulsidas and Sant Raidas; and in the modern era, Chaitanya Mahaprabhu and Mirabai — these are some celebrated exemplars of Bhakti Yoga.
For the attainment of the Nirakara (formless) God, Dhyana Yoga and Jnana Yoga are the special paths. Dhyana Yoga is suitable for those who can keep their body still in one posture without movement, and who have the inclination and capacity to concentrate their mind on a single point for extended periods. The path of Jnana is for those seekers who possess greater intellectual capacity and whose inclination is toward the Nirguna-Nirakara God. They are inclined and capable of studying and reflecting upon the scriptures and seek to realize the metaphysical nature of God.
On the other hand, Bhakti Yoga is the most suitable path for those who are emotion-centered and who delight in rasa (spiritual sentiment). But this does not mean that the path of Bhakti is only for the less intellectually gifted. This path can also be dear to the intellectually endowed and faithful — and the Lord Himself has declared in the Gita that the wise devotee (jnani-bhakta) is especially dear to Him (7:17).