ROME—The Vatican’s arrest of one of its diplomats on suspicion of violating child-pornography laws has set the stage for the first trial for that crime in Vatican City—an event likely to be closely watched as a sign of Pope Francis’ commitment to combating clerical sex abuse.

“Let us hope there is a trial soon and it is transparent,” Marie Collins, a former member of the pope’s advisory panel on child protection, tweeted Sunday. “The possession of child-abuse images is a serious crime.”

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