Complex sentence: The ancient Indians recognized Agni as the power of heat and light and the will-power united with wisdom, they knew the human will-power to be a feeble projection of this power which they believed could be strengthened by the Rig Vedic chants to Agni.
Split into simpler sentences: The ancient Indians recognized Agni as the power of heat and light and will-power united with wisdom. They knew human will-power to be a feeble projection of this power which they believed could be strengthened by the Rig Vedic chants to Agni.