Sanatam dharma is the eternal religion

Sanatan dharma is made up of two sanskrit words, sanatan and dharma. The word Sanatan means eternal and dharma means religion, law or duty. So sanatan dharma means the religion. 

Sanatan dharma is eternal law of life

Sanatan Dharma is the fountain head of all the religion.

Sanatan dharma is veritably the fountain-head of all religions. It contains within itself the seeds of
all religions. It includes all religions and excludes none.