There were twelve Alvars who appeared in South India. Not all at the same time, but over a period of several centuries. They established the basis of the Krishna bhakti cult in the Kali-yuga. The appearance of such great devotees in the Kali-yuga is predicted in the Srimad-Bhagavatam. Srimad-Bhagavatam was spoken at the beginning of the Kali-yuga, and when Krishna left this planet then he took with Him dharma. The Vedic dharma at that point disappeared, or became invalid, and spiritual knowledge was also obscured. But it says in the same verse that Lord Krishna left the Srimad Bhagavatam for the people in Kali-yuga to get light out of. Now still, the book Bhagavata was there but they also needed the person Bhagavata, or one who lives the Srimad-Bhagavatam. In other words, they needed the spiritual master. So in the initial stage of Kali-yuga, the first few centuries, these twelve Alvars appeared in South India, and actually established the basis of what would later on become the four Vaishnava sampradayas. The four sampradayas all had their origin in South India, and the founders of these sampradayas each in their own way drew, to a greater or lesser extent, from this tradition of the Alvars, especially in the Lakshmi sampradaya, but it is also there in our sampradaya too, and in the others. The Radha Krishna cult is the further development of the devotional tendency of loving God in close fellowship and in the spirit and relation of a woman to her husband or lover. This tendency is striking in the Prabandham of the Alvars. Goda, the famous woman Alvar, is said to have been married to the Deity Ranganatha of the Sri Rangam temple. Tondaredipodi Alvar (Bhaktanghri Renu in Sanskrit) expresses in his Tirup Palliyeducci (Paramatmar Jagarana in Sanskrit) that to serve and love God one's spiritual body is the summom bonum of one's service to God.
From Sri Atmattava Dasa