Devotionalism goes back to the beginning of Buddhism:
The Buddha taught that the biggest barrier to realization is the notion that “I” am a permanent, integral, autonomous entity. It is by seeing through the delusion of ego that realization blooms. Devotion is an upaya for breaking the bonds of ego.
For this reason, the Buddha taught his disciples to cultivate devotional and reverential habits of mind. Thus, devotion is not a “corruption” of Buddhism, but an expression of it. Of course, devotion requires an object…
For more discussion of this kind of devotion, see the essay ”Devotion in Buddhism“ by Nyanaponika Thera.
Devotion in Buddhism
by Nyanaponika Thera
Here is Thanissaro Bhikkhu explaining how he came to the realization that devotional practice is a legitimate expression of Buddhism, rather than the corruption of it:
What are the discoveries that have broken the frame? Among the most interesting rediscoveries are some very early texts known as the avadanas [lessons]. These are stories about how the Buddha and famous arhats [those who have attained the penultimate stage of awakening] got started on the path many aeons ago. Like the jatakas [tales of the Buddha’s past lives], the stories are aimed at inspiring a sense of devotion.
A lot of these texts were written when the stupa cult was becoming popular in India. They were advocating the idea that in order to get started on the path one needs to have the merit field of the Buddha. By performing services to the Buddha or his relics, you plant the seeds of merit that will eventually result in awakening.
The avadanas changed my understanding of some the rituals and ceremonies I experienced as a monk in Thailand. Until the rediscovery of the avadanas, it was assumed that popular devotional Buddhism as practiced in Southeast Asia today was a corrupted form. But looking at the avadanas you see that the practices are in fact very old.
These texts, with their emphasis on Buddha-fields and vows for awakening, also provide the missing link between the early canons and the Mahayana, thereby rewriting the story of how the Mahayana arose.