Buddhist categorizes all other religions as ‘foreign path’ (外道) and repels them as ‘not the true path’. However, not many people willing to look into the fact that the origin of Buddhism is closely related to the so-called ‘foreign path’ Brahmism. Hence it is a fact that before one ventures into Buddhist Mantrayana, he/she should first learn about Brahmism. In another words, Brahmism is the door to a better understanding of Buddhism.
What Brahmism advocates is termed as ‘the worldly matters’ (世间法) in Buddhism. This type of teachings can be readily accepted by the general public; on the other hand, what the Buddha advocates, are a set of more abstract philosophy which not many people can or willing to accept.
The worldly matters can be categorized as common sense (of that time): medicine, astrology, magic rituals and other ritual workings. These common senses are the basis of an ideal world. This is what is advocated by Buddhism as ‘non-worldly matters’ (出世法). The four Vedas (Rig, Sama, Yajur and Atharva) which formed the basis of Brahmism; they are also basis of Buddhism, especially the Mantrayana. Hence, the gods and goddesses of Veda are also gods and goddesses of Buddhism.
For example: Shiva (Mahesvara), Bodhisattvas, Bhagavan, Yaksa, Raksa, human and non-human etc. are same in Brahmism as well as Buddhism. Some Buddhists say that there are Buddhist Mahakala and non-Buddhist Mahakala; obviously this is just ‘character robbery’ and not true and unacceptable.
When we look further into theoretical teaching of Mantrayana and Brahmism (教相): The 5 Tattvas: earth, water, fire, wind and space are similar if not identical. It is certainly true that, in Buddhist theory, an additional aspect of ‘consciousness’ (识) is added at a later date forming 6 Tattvas.
If we further investigate the ritual aspects of Buddhism and Brahmism, both of these religions likewise embrace: pacification, increase benefit, love, subjugation and summoning rituals. Along with the rituals come with: offering substances, herba, medicine, woods, flowers, fruits, cow 5 pure substances etc; in addition the ritual implements such as vajra, magic sword, bell, kila etc. Also in order to accomplish various Siddhis, actions such as fasting, meditation, forbiddance to talk etc are also part of the rituals in the hope to be reborn into particular heaven or it is merely to obtain benefits of this life. All of the above mentioned almost entirely belonged to the curriculum of Brahmism.
It is my personal opinion that Buddhism, especially the Mantrayana is not a standalone religion. The reason is that after we have excluded elements of Brahmism, what are left to Buddhism are only theory and more theories.