The cleric, a defender of the poor and vocal critic of the military in El Salvador, was shot dead in 1980 while celebrating mass.
He had delivered a sermon the day before urging rank-and-file soldiers not to carry out orders which would lead them to become involved in human rights abuses. Noone was ever convicted of the murder.
Declaring him a martyr clears the way for him to be beatified -- a move which would lead to him being referred to as "blessed" and can lead to sainthood.
Romero's support for the oppressed has seen him held up by some as a champion of Liberation Theology, a political movement rooted in South America which advocates the Church working with the poor to bring about social change.
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While Romero was not actually a subscriber to the theology, his support for the oppressed meant some within the Church were reluctant to beatify him due to concerns his death could be exploited for political or ideological reasons.
In August he described the Salvadoran bishop as a "man of God" whom he hoped would be quickly beatified.