During fascist rule

On 24 December 1938 he was appointed as the Bishop of Alba Iulia in a decree that Pope Pius XI signed. He received his episcopal consecration in 1939 just after that pope's death from Andrea Cassulo before he was enthroned in his new episcopal see. Márton was one of the first intellectuals to oppose the preparations for World War II in public and he had started this in 1938 prior to his appointment as a bishop. He remained in the southern part of Transylvania, which remained part of Romania following the Second Vienna Award in 1940, unlike the northern part, which became part of Hungary. In a speech he gave in Saint Michael's Church – during a visit to Kolozsvár on 18 May 1944 (for the ordination of three new priests) – he condemned the prepared deportation of the Romanian and Hungarian Jews. That week on 22 May he also wrote letters to the Hungarian government as well as to the local police and other authorities requesting the prohibition of the deportation. But the response to his pleas was his expulsion from Alba Iulia.^[3]

During communist rule

In 1945 – after the death of Cardinal Jusztinián Serédi – Pope Pius XII wanted Márton to become the next cardinal for the Hungarian nation, therefore leading Serédi's vacant see. But the opposition of the Hungarian Communists saw another prelate chosen for the position.

Márton continued to be a strong advocate for religious freedom and human rights which made him an opponent of the Romanian Communists and of the dictatorial regime that it had established in late 1947. The bishop was arrested on 21 June 1949 and was sentenced to life imprisonment in mid-1951; but in 1953 – with the change of the political climate in Romania – the circumstances of his imprisonment became more bearable.^[1] Pius XII in 1949 (while Márton was jailed) gave him the personal title of archbishop ("ad personam"), though he made no public announcement of this and informed Márton in a private letter. Márton was moved into a villa in the outskirts of Bucharest but was not allowed to have contact with the outside world. He was released in 1955.

Márton decided to revisit his bishopric after his release where he received a grand welcome on the part of an enthusiastic crowd. This proved embarrassing for the authorities and he was confined to house arrest following this. Márton was not allowed to leave the bishopric building for the decade and was permitted to emerge later in 1967. He was released as a result of negotiations that Cardinal Franz König had conducted in Bucharest. But Márton was still cautious of possible harassment from the authorities and volunteers often accompanied him on his pastoral visitations to ensure he was safe and left alone. He served as the President of the Romanian Episcopal Conference from 1970 to 1980. In 1972 he was diagnosed with cancer. He travelled several times to Rome where he met with Pope Paul VI.^[1]