Achievements of Kanishka bought the spreading of Buddhism after Ashokan period. Kanishka, the famous king of the Kushana dynasty, was born of mixed race. But he became an Indian because of his long acquaintance with the people and culture of the land. Kanishka transformed into a devoted Buddhist during the course of his reign. The coins of Kanishka pointed to the gradual alteration of his religious beliefs initiated with pantheism and culminating in adoption of Buddhism. There are various legends depicting the story of Kanishka`s conversion.

Religious Policy of Kanishka
Historians depend on the legends to determine the religious policy of Kanishka. According to such a legend, the influence of the great scholar Asvaghosa led to the conversion of Kanishka. After occupying Pataliputra, Kanishka came in contact with Asvaghosa. Kanishka was quite impressed with the teachings of Asvaghosa. Asvaghosa accompanied him to Peshawar and introduced him to the Buddhist creed. Though a dedicated Buddhist himself, Kanishka was not a bigot. He was equally tolerant towards the other religious creeds, which are evident from the figures of various Hindu, Persian and Greek deities, found on his coins. Some scholars however suggests that these figures are not conclusive proofs of the religious toleration of Kanishka, rather the coins can be considered merely a token, which indicates the diversity of religious beliefs and ideas within his Empire.
The reign of Kanishka left a memorable mark in the history of ancient India, particularly for his patronage to the religion of Sakyamuni. Ashoka devoted his life for the spread of the Hinyana creed while Kanishka devoted himself to the service of the Mahayana creed. Different scholars have approached different theories regarding the actual date of Kanishka`s conversion into Buddhism. Some scholars are of the view that Kanishka embraced Buddhism during the first era of his reign. Sten Konow, a Norwegian Indologist, had come to this conclusion after reading a date in the Peshawar casket information. B.N Mukherjee, an Indian historian, has suggested that the Peshawar casket information did not bear any date. However, it is generally believed that Kanishka embraced Buddhism after his invasion of Pataliputra, where he came under the influence of the Buddhist scholar Asvaghosa.
Fourth Buddhist Council Under Kanishka`s Rule
During Kanishka, the famous Fourth Buddhist council was convoked at Kundalavana Vihara. Though there are enough controversies among the scholars about the actual location of Kundalavana Vihara, it is generally believed that Kundalavana Vihara was located in Kashmir. A selected body of scholars participated in the council. The fourth Buddhist council was organised under the patronage of Kanishka, which was headed by scholars like Vasumitra and Asvaghosa.
The Convocation of the first Buddhist council marked the
ascendancy of the Mahayana Buddhism with Sanskrit as
its vehicle of propagation. Kanishka regarded Mahayana as his state religion.
The state religion of Kanishka determined preaching the divinity of Gautama Buddha and the worship of Buddha’s
image had a greater appeal compared to the dry, scholastic Hinyana creed. Mahayanism, the state religion of Kanishka preached
not only the divinity of Buddha and worshipped the image of Buddha, but they
also preached the efficacy of prayer, devotion and faith. Salvation of
entire universe was the idea of Mahayanism. Moreover, the medium of propagation
of Buddhism was Sanskrit,
which had some social significance. Kanishka undertook this step because at
that time Sanskrit was considered an aristocratic language, which was more ornamental
compared to the spoken language Pali. Therefore, with the use of Sanskrit as
the medium of propagation, Kanishka intended to alienate Buddhism from the
common masses.
Kanishka in his religious policy of Mahayanism discarded the doctrine of personal salvation by penance and ethical practices in order to be "self-illuminated". The religious policy of Kanishka stated that Bodhisatva would work for the salvation of all beings. Hence during the reign of Kanishka, people did not need to undergo hardship and penance in order to attain salvation. Kanishka replaced the worship of Buddhist Symbols like Buddha`s footprints, Dharmachakra, Stupa or Bodhi Tree by initiating the worship of Buddha and Bodhisattva images.

Buddhism during Kanishka's Rule
The cult of Bodhisattva and the worship of Buddha images became widely prevalent in the Kushana period. One of the earliest figures of Buddha are found in Kanishka’s coins and in the Peshawar casket. The term "Bodhisattva" had a dual significance as it was upheld by the religious policy initiated by Kanishka. In one sense it meant a person worked for the salvation of the mankind. In the second sense it meant the previous incarnation of Buddha. Kanishka also undertook the policy of propagating his religion to the distant countries including Tibet, China, Burma and Japan. Historians have later presumed that since Kanishka had commanded over the major parts of Central Asia, his own creed of Mahayanism was spread to those countries. From the accounts of Hiuen Tsang and Al Beruni, it is known that Kanishka constructed the great relic tower at Peshawar, which was famous throughout the Buddhist world. The Greek architect Agelisas or Agisala constructed the tower. Mahayana Buddhism, as the state religion of the Kushanas received a great impetus.