I have taken notes of Georg Feuerstein’s summary History of the Bhakti Ideal article from his massive The Yoga Traditions, pages 39-41. (Hohm Press, 1999 and 2001)

NOTES on Bhakti from The Yoga Tradition by Georg Feuerstein (Hohm Press). From subsection “The History of the Bhakti Ideal”.
Page 39 notes: ~History known imperfectly. ~Only few hymns in Vedas showing passionate, emotional relationship to invoked deity. ~Vedic invocations “lofty but aloof, lacking the devotional pathos typical of the medieval bhakti literature.” ~Examples: praise of Agni in opening hymn for Rig Veda ~a hymn also praising Indra ~Vedic devotionalism in context of elaborate sacrificial religion ~over time that sacrificial ritualism became more detailed and demanding and suppressing therefore the devotional energy ~existing alongside the primary Vedic religion was the monotheistic Pancaratra tradition, followed by those unconvinced of the Vedic pantheon or finding the impersonal Absolute too abstract, or emotionally dissatisfied with the Vedic sacrificial ritualism. ~Pancaratra tradition attracted those longing for personal intimacy with Divine. ~Worship focused on God “Vasudeva-Narayana-Vishnu”. Page 40 notes: ~ This tradition existing before time of Buddha and was widespread “at the margins of the ancient Indic society” and was impacting the orthodox somewhat despite being looked down upon.

Page 40 notes: ~ “It was largely because of the success of this religion-spiritual tradition—epitomized in the immense popularity of the Bhagavad-Gita and the Bhagavata-Purana—that Hinduism came to be what it is today: a religious culture of temples, sacred imagery, and devotional worship. The Pancaratra tradition, which is sometimes referred to as Bhagavatism, also was instrumental in the post-Vedic development of Yoga. It introduced the concept and practice of bhakti into what was all too often a somewhat heady or dry approach to Self-realization.” ~bhakti path originally most associated with worship of God Vishnu but term bhakti is “used in the technical sense” around 4 BCE or earlier in monotheistic work called Shvetashvatara-Upanishad, dedicated to God Shiva.

Page 40 notes: ~the Schvetashvatara-Upanishad (which Feuerstein sees as being earlier, pre-Buddha, than the standard 4C BCE time period) “introduces the dual idea of love for God and love for the spiritual teacher, who should be loved in the same manner as the Divine, since he or she is its embodiment.” ~Monotheistic teachings in Vaishnamism (mostly Pancaratra tradition) with Vishnu as “Divine Person” and Krishna most frequently worshipped Incarnation. And, in Shaivism (worship of God Shiva. ~ both Shiva and Vishnu mentioned in Rig-Veda and likely had worshippers but identifiable religious movements of Shiva or Vishnu worship were only apparent “during the second half of the first millennium BCE.

Page 40 notes: ~early Shiva sects: Pashupatas, Kapalikas, Kalamukhas (Chapter 11). ~a THIRD development with roots in Rig Veda was Shaktism, worship of Divine “in its feminine or power aspect–as Shakti”. ~Bhakti plays major and important role in this movement (ritual worship of the Goddess) ~names of Goddess includes: Mahadevi, Kali, Durga, Parvati, Annapurna, Candi, Sati.

page 40 notes: ~ beginning of CE, Shaktism merged with Tantra without losing own identity. ~~Bhagavad Gita, Vaishnava scripture of 6th Century BCE, used bhakti term liberally. ~Means: proper relationship between Divine and practitioner; goal of liberation; Vishnu worship (often in form of Krishna) “became increasingly popular in the early centuries of the Common Era, attracting large numbers of adherents in both North and South India.”

page 40-41 notes: ~ bhakti already playing important role in the Shiva devotees creating the Shaiva-Siddhanta system in South India before the C.E. ~ Tiru-Murai (700 C.E. but attributed to 200 B.C.E-100 C.E.) called Tamil Shaiva equivalent of Vedas from north India. ~ Alvars, Vishnu devotees, minority in south India and promoted by 12 bhaktas singing devotional praises in the 7th or 8th Century C.E. ~ northern versions of Shiva and Vishnu worship distinctly different in ways ~ Alvars followed by Acaryas (Preceptors) who focused on systemizing “the monotheistic theology of Vaishnavism”. ~ most notable of Acaryas was Ramanuja (1017-1137 C.E.), a southern Brahman who taught “qualified non-dualism” (making him a pioneering figure equal to Shankara) ~ Ramanuja made “the idea of a suprapersonal divine Being logically consistent with the teachings of Vedantic non dualism.” ~ he also integrated the northern and southern traditions.

page 41 notes: ~ Ramanuja’s work paved the way for medieval bhakti-marga, or ‘way of devotion’. ~ Ramanuja felt there was no end to spiritual practice, that devotion was not only the means to liberation but the primary goal of all spiritual effort. ~ history of bhakti approach “vastly complex” with the traditions of south India neglected [in studies, I presume]. ~ teachers of this tradition say it is easiest path to liberation. ~ “Loviing devotion to the Lord bears fruit readily when it is constant, unswerving, and purposeless.” ~ Gopis of Krishna legends perfect example.