In the Vedas, Karma Marga (path of action) and Gayn Marga (path of knowledge) are described. Upanishads (Vedanta or the end part of the Veda) talks about the path of knowledge, while the earlier Vedic literature gives stress on performing different religious rites (Karma-Kanda). Bhakti was not part of the Vedic tradition. Though, in the Bhagavad Gita (especially at the end part) we see the inclusion of Bhakti tradition. In later Hinduism, especially in Vaishnavism, Bhakti becomes mainstream. How Bhakti marga suddenly found its place in Hinduism?
Bhakti movement, which is most prevalent in Vaishnava sect cannot be traced back to the Vedas. Rigvedic God Vishnu had initially nothing to do with this. This is a pre-Vedic non-Aryan cult which later got fused with Vedic religion. This is how it happened.
At the end of the Upanishadic period (8th to 6th centuries BCE), Vedism was in decline. It was a period of great philosophical ferment in India. The religious landscape of India was slowly changing. Heretic schools like Buddism and Jainism were rising. At the crucial juncture, Bhakti movement started.
Initially, there were many sects and they gradually got fused and form a single sect.
The sect of Vasudeva (Vasudevism) - Vasudeva was a hero of the Vrsni tribe. This cult centred around devotion for the deified Vasudeva. Vasudevism is the earliest form of Vaisnavism. It got stabilized around 6th to 5th centuries BCE (time of Panini). The tradition continues uninterrupted from that time. Bhagavad Gita was written in 3rd Century BCE. In Bhagavad Gita, Krishna declares that of the Vrsnis he is Vasudeva (10.37).
The sect of Krishna (Krsnaism) - Another theistic cult centred around the deified hero Krsna, who was the religious leader of Yadava tribe. He also most probably the same person mentioned in the Chandogya Upanishad as the pupil of Rsi Ghora Angirasa.
These two tribes were related. Eventually, they came closer due to political pressure. Two divine personalities merged and gave rise to supreme God Bhagavan Vasudeva-Krsna. The third current of Gopala Krsna originated in the cowherd communities of Abhiras later also got integrated. Non-Vedic Nara-Narayana cult originated in Badri (Hindukush region) also got assimilated.
As this Krsna movement has non-Vedic origin and development, later, it sought affiliation from Vedas. Now the Vedic God Vishnu of Rigveda got assimilated into newly formed Krsnism. Vishnu who was a subordinate god in the Rigveda now got elevated to supreme godhead. Bhagavad Gita is the synthesis which shows both Vedic and Vasudeva traditions and depicts Krsna as supreme Lord. During the Gupta period, who were devoted Vaisnavas, Vaisnavism took its final shape. Vaisnava Puranas were written.
Alvars - The next revival of Vaisnavism happened around, 6th to 9th centuries CE, in South India. Tamil poets Alvars wrote intensely devotional songs for Vishnu. The bhakti tradition of Alvars later got mainstream by getting accepted by the Vedantic tradition of Ramanuja and formed Sri Vaisnava school. Bhagavata Purana was written around the 10th century CE. This book basically adopted Alvar devotion (translated and paraphrased Alvar poems) and gave it a Sanskrit mold. Bhagavata Purana is the most authoritative text in Vaisnavism. Through Bhagavata Purana, the intense devotion of Alvars greatly influenced entire Hinduism.
Caitanya - Later northern and central India also experienced a revival of Vaisnavism around 13th to 17th centuries CE. Most notable of North Indian Vaisnava movements is the Bengal school of Vaisnavism formed by Caitanya. Now, ISKCON (founded by Prabhupada) popularized it not only in every part of India but around every corner of the world. In Ghana (Africa), Hinduism is the fastest growing religion, credit goes to the constant preaching by ISKCON.
- Vaisnavism - an overview by R. N. Dandekar, 1987. Lindsay Jones Encyclopedia of Religion. 2nd Ed. (pp: 9816-9819).
- Alvars by Friedhelm E Hardy, 1987. Lindsay Jones Encyclopedia of Religion. 2nd Ed. (pp: 430 - 431).