Very briefly Sanatana Dharma is a many-sided approach to the One Supreme Reality behind and within everything and, the ways to lead our life so that we can discover and manifest this Reality in our everyday life from the highest aims to the most mundane details of our existence.
Its fundamental truths are as follows.
1. There is One Supreme Reality that people call by different names. This is the truth of Vedanta.
2. There are many approaches to this Reality and as is the approach so is our experience of IT
3. There are many powers and aspects of this Reality since it is One and Infinite. These aspects and powers of the One are known as the gods.
4. This Supreme Reality transcends the creation as well as is Immanent in each particle of it
5. It is given to man to discover / uncover this Reality and becoming one with IT transform his earthly existence.
6. Creation is a progressive manifestation of this Reality that is the essence of everything and dwells in everything as its seed and core. This progressive manifestation is what we call as
evolution.
7. It is only by discovering and becoming one with this Reality that our life can find its true purpose and fulfilment.
8. This Reality is eternal, unchanging and universal (Sanatana) and does not depend upon anything though everything depends upon IT.
9. It has both static and dynamic aspects. Static IT reveals Itself as the Lord, Ishwara who is the Source and Witness of Creation. Dynamic IT reveals Itself as the Divine Mother (Shakti, Maya, Prakriti) that builds forms for the Lord to dwell in and endows everything with some quality or the other of the Lord (name). The creation therefore is not cut off from the Creator, rather He extends into each and every element of creation. The world is an objectivisation of the Divine. The static aspect is the Purusha of Vedanta. The dynamic aspect is the Shakti or Devi in the tantra.
10. This unfolding and manifesting of the Supreme Reality in creation, in the cosmos as well as the individual is a complex and many-sided process that involves a host of forces. Aligning with the unfolding in the right way is Dharma. Opposing this unfolding and resisting or trying to block or impede it is Adharma.
These are some of the fundamental truths of the Sanatana Dharma around which grew the way of life that we know today as the Hindu culture.