The treason of having religious beliefs that were incompatible with the Roman religion.
The "very tolerant' Romans:
In Egypt, some
Manicheans, followers of the prophet
Mani, were denounced in the presence of the proconsul of Africa. On March 31, 302, in a rescript from Alexandria, Diocletian, after consultation with the proconsul for Egypt, ordered that the leading Manicheans be
burnt alive along with their scriptures.
[115] This was the first time an Imperial persecution ever called for the destruction of sacred literature.
[116] Low-status Manicheans were to be executed; high-status Manicheans were to be sent to work in the quarries of Proconnesus (
Marmara Island) or the mines of
Phaeno. All Manichean property was to be seized and deposited in the imperial treasury.
[115]
On February 23, 303, Diocletian ordered that the newly built Christian church at Nicomedia be razed,
its scriptures burned, and its treasures seized.
[127] February 23 was the feast of the
Terminalia, for
Terminus, the god of boundaries. It was the day they would terminate Christianity.
[128] The next day, Diocletian's first "Edict against the Christians" was published.
[129][notes 14] The key targets of this piece of legislation were senior Christian clerics and Christians' property, just as they had been during Valerian's persecution.
[133] The edict prohibited Christians from assembling for worship,
[134] and ordered the destruction of their scriptures, liturgical books, and places of worship across the empire.
Diocletian requested that the edict be pursued "without bloodshed",
[144] against Galerius's demands that all those refusing to sacrifice be burned alive.
[145] In spite of Diocletian's request, local judges often enforced executions during the persecution, as capital punishment was among their discretionary powers.
[146] Galerius's recommendation—burning alive—became a common method of executing Christians in the East.
[147] After the edict was posted in Nicomedia, a man named Eutius tore it down and ripped it up, shouting "Here are your Gothic and Sarmatian triumphs!" He was arrested for treason, tortured, and burned alive soon after, becoming the edict's first martyr.
[148][notes 16] The provisions of the edict were known and enforced in Palestine by March or April (just before
Easter), and it was in use by local officials in North Africa by May or June.
[150] The earliest martyr at
Caesarea was executed on June 7,
[151] and the edict was in force at
Cirta from May 19.
[152] The first edict was the sole legally binding edict in the West;
[153] in Gaul and Britain Constantius did not enforce this edict
[154] but the East progressively harsher legislation was devised...
In 304, the fourth edict ordered all persons, men, women, and children, to gather in a public space and offer a collective sacrifice. If they refused, they were to be executed.
I didn't. I said the Romans killed them for following the wrong religion: one that was incompatible with theirs, just like Christians did at times.
“You should not only worship the divine everywhere and in every way in accordance with our ancestral traditions, but also force all others to honour it. Those who attempt to distort our religion with strange rites you should hate and punish, not only for the sake of the gods … but also because such people, by bringing in new divinities, persuade many folks to adopt foreign practices, which lead to conspiracies, revolts, and factions, which are entirely unsuitable for monarch.” Dio Cassius - History of Rome
I'm guessing you wouldn't engage in apologetics to defend Christians if they were the oppressors rather than the victims. Funny that.