In many Rig Vedic Manthras this is clearly announced:

Ekam Sath viprraa bahudaa vadanthi
Agnim yamam maathariswaanamaahuh

"There is only just One: those who have seen the Truth praise it in many ways: Agni Yama Maathariswaan". This Brahmam, the One without a Second, is designated in the Rig Veda as Hiranyagarbha, Prajapathi, Visvakarma Purusha. The Hiranyagarbha Suktha and the Purusha Suktha are classical examples of this vision.

The way of living of the ancient Aryans is designated "Dharma". It can also be called, "Yajna". Their daily disciplines were marked by rituals, puja and praise, surrender and dedication. So, their life was full of Bhakthi or Devotion. The word Bhakthi might not be used, as such, in the Samhithas, but is not the word "Sraddha" found?

Sraddhayaagnih Samidhyathe Sraddhaahooyathe havih
Sraddhaam bhagasya moordhanivachassaa Vedayaamasi

"It is only through Sraddha that the Flame of the Sacrificial Fire is lit and fed. It is through Sraddha alone that the offerings reach the Gods who are called. Let us praise Sraddha, which is the highest form of worship". See, how mighty is the power of Sraddha!

The simple spontaneous disciplines of the Vedic Age gradually became complex and confused by the overgrowth of rituals and formal rules; with the passing of time, it was declared that Dharma consists of Yajna and Homa, that Heaven can be gained only by the performance of such rites! Though the Yajna was really a method of worship of the Gods, value was shifted from the Gods, to the Yajna itself. "The Gods were only the means; those who desired Heaven must do Yajna". Such was the turn the declaration took.