The most direct evidence for Kushan religion is small and portable. The dynasty's coins carry, on their reverses, a remarkable range of deities with their names written beside them: Iranian figures such as Mithro, Mao and Nana, Greek ones such as Helios and Herakles, Indian ones including Shiva under the name Oesho, and the Buddha, labelled Boddo, in a very rare early instance of the Buddha on a state coinage.
No other ancient state issued anything comparable. What it means is argued. The coins may record a genuinely plural religious policy, a court that patronised many communities, a ruler's personal devotions, or a sequence of shifts across a long reign, and the same evidence supports more than one of those. What is not in doubt is the geography behind it.
The Kushan realm ran from Bactria across Gandhara into northern India and sat on the routes between the Iranian world, the Indian world and the trade going east, so a coinage that had to circulate among all of those populations had reasons to speak several religious languages at once. The atlas records the deities as attested and the underlying policy as debated, and notes that the Kushan royal sanctuaries known from Bactria, with their dynastic images and inscriptions in an Iranian language written in Greek letters, show the same mixture of inheritances in another medium entirely.