Can I practice Buddhist and non-Buddhist teachings?

If we recognize that worldly gods are not suitable objects of refuge and then make the promise, ‘I will not go for ultimate refuge to objects other than the Three Jewels’, this is going for refuge by abandoning going for refuge to other objects. If we go for refuge to teachers whose instructions contradict those of Buddha we shall be led onto wrong paths; and if we go for refuge to worldly gods, although we may receive some short-term benefits we shall be diverted from perfect paths leading to liberation and enlightenment. However, when we abandon going for refuge to other objects we should not do so out of sectarianism. Our motivation should be simply to keep our refuge vows purely and avoid the harm we would inflict upon ourself by breaking our commitments. If we have perfect faith in Buddha there is no reason why we should ever need to go for refuge to other objects.

Some people with pure faith can make the promise: ‘I will go for refuge only to the Three Jewels.’ This promise is very beneficial but it will be stronger and more stable if it is based on faith gained through logical conviction. The famous Buddhist scholar Todzun Drubche was formerly a non-Buddhist, and highly trained in non-Buddhist scriptures. He went with his brother to Mount Kailash, which was regarded by non-Buddhists as the palace of Ishvara. When he arrived there Todzun Drubche discovered that even Ishvara was going for refuge to Buddha and so he developed faith in Buddha and trained in Buddhist scriptures. He later composed a text called Praising Buddha as a Teacher Far Superior to Any Other, in which he wrote:

O Buddha, I go for refuge to you, abandoning all other teachers. What is my reason? It is that you alone are completely free from all faults and endowed with all good qualities…. The more I compare non-Buddhist and Buddhist teachings, the stronger my faith in you grows.

Todzun Drubche was able to develop strong faith in Buddha based on logical conviction because he realized that Buddha has abandoned all faults and perfected every good quality. From his experience of non-Buddhist teachings he could understand clearly the superiority and profundity of Buddha’s teachings. He realized that some non-Buddhist teachings posit a permanent, partless, independent self, and that any teaching supporting this view supports self-grasping ignorance, which gives rise to all the actions that cause samsaric rebirth.

In Praise to Dependent Relationship Je Tsongkhapa says of Buddha:

Realizing the way things exist, you taught it well.
Learning and practising what you taught
We abandon all faults
Because you have shown how to cut their root.

Anyone who diligently relies for a long time
Upon teachings that contradict your own
Invites many faults
Because those teachings nourish self-grasping.

O Wise is the scholar who understands
The difference between these two.
How could he fail to develop faith in you
From the depths of his heart?

(Geshe Kelsang Gyatso, Joyful Path of Good Fortune: the Complete Buddhist Path to Enlightenmentpp. 212-213, © 1990, 1995)