You must be Buddhist because of how the rainbow body process works.
The rainbow body process is based in dependent origination and dedication of merit, proper view etc.
Many of the elements necessary for rainbow body are combined into one little paragraph such as:
I take refuge in the Buddha, Dharma and Sangha.
By the merit I accumulate by practising generosity and the other perfections, I attain perfect awakening for the benefit of all sentient beings.
In this one paragraph, you have dedicated merit for the purpose of obtainining Buddhahood for all sentient beings. Now pay attention. Since there is an infinite number of sentient beings, your aspiration becomes infinite, thus allows you to obtain perfect Buddhahood.