Who was the first Christian cleric to be executed for religious reasons by the now Christian Roman Empire in the late 4th century?

Priscillian (born c. 340, Spain-died 385, Trier, Belgica, Gaul [now in Germany]) was an early Christian bishop who was the first heretic to receive capital punishment. A rigorous ascetic, he founded Priscillianism, an unorthodox doctrine that persisted into the 6th century.

In AD 155, the beloved Bishop Polycarp of Smyrna was martyred after a mob cried out for the 'atheist' to be killed.
He famously refused to renounce his commitment to Christ and was burned at the stake.