This section is from the book "Some Contributions Of South India To Indian Culture", by S. Krishnaswami Aiyangar. Also available from Amazon: Some Contributions of South India to Indian Culture.
We have already shown under the section bhakti that the rudiments of Vaishnavism in the ordinary form and in the agamaic are traceable in the classical literature of Tamil. This form of bhakti had a course of development on much the same lines as Saivism in the Tamil country. In general outline, this would involve the substitution of Vishnu for Siva as "the soul-saving supreme being" whose grace is of paramount importance to the attainment of salvation by an individual. We have already pointed out that the most popular form in which Vishnu figures in early Tamil literature is that of several of his avataras of the pauranic character. It would be easy to quote references from the early classics to the avataras (descents of God) Rama and Krishna even in secular literature. Along with this, simultaneously, we find indubitable references to the agamaic four Vyuhas. But the idea of Vishnu is of course indicated by the general name Tirumal for the god. We thus find that, almost at the beginning of the Christian era, the features of northern Vaishnavism in all its variety were prevalent in the south. It is the course of further development of Vaishnavism that lends character to the movement and makes it a distinct feature of South Indian history.