**Chapter 12 of Bhagvad gita is named "Bhakti yoga"** .  It describes path of completely loving devotion to God in very detailed manner.  Bhagvad gita is among Prasthantrayi scriptures of Vedanta Philosophy. So **that obviously means Bhakti yoga is atleast as old as Gita or even far more older. that definitely pushes it hundreds/thousands years before Christ**

see : 

- https://www.esamskriti.com/e/Spirituality/Bhagavad-Gita/Bhagavad-Gita~Chapter-12-Bhakti-Yogah~Yoga-Of-Devotion-1.aspx
- http://www.bhagavad-gita.org/Gita/chapter-12.html

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Now, [This book "Bhakti yoga for beginners"][1] ; shows that bhakti yoga is dealt by Sama veda.


  [1]: https://books.google.co.in/books?id=EYseAgAAQBAJ&pg=PA13&lpg=PA13&dq=bhakti%20yoga%20veda&source=bl&ots=1sPUqaSi91&sig=4uTuTIkDCB6MWRPwHwYtmyYDG98&hl=en&sa=X&ved=0ahUKEwj65L-_0JPbAhVEPY8KHVmhC344ChDoAQhEMAU#v=snippet&q=bhakti%20yoga%20veda&f=false