Earliest references of bhakti:
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
Earliest references of bhakti:
Now, This book "Bhakti yoga for beginners" ; shows that bhakti yoga is dealt by Sama veda.
Apart from that, term Bhakti appears also in the Shvetashvatara Upanishad:
यस्य देवे परा भक्तिः यथा देवे तथा गुरौ ।
तस्यैते कथिता ह्यर्थाः प्रकाशन्ते महात्मनः ॥
He who has highest bhakti of Deva (God),
just like his Deva, so for his Guru (teacher),
to him who is high-minded,
these teachings will be illuminating.
(Shvetashvatara Upanishad 6.23)
Note that Shvetashvatara Upanishad is embedded in Yajurveda.
Earliest references of bhakti movement:
Regarding bhakti "movement", the link which you refer only talks of "movements" in context of modern day or middle era; and that too as per western interpretation. Western interpretations deliberately want to show that bhakti movement of India is post christ era only. But In ancient era, narad, hanumana(post-ramayana war) were themselves shown as bhakts roaming and chanting around everywhere. These examples and also Panchratra agama indicates powerful presence of bhakti "movement" existing before Buddhist era.