linguistic:paraphrasing
Early operatic career in Greece

After several appearances as a student, Callas began appearing in secondary roles at the Greek National Opera. De Hidalgo was instrumental in securing roles for her, allowing Callas to earn a small salary, which helped her and her family get through the difficult war years.

Callas made her professional debut in February 1941, in the small role of Beatrice in Franz von Suppé 's Boccaccio. Soprano Galatea Amaxopoulou, who sang in the chorus, later recalled, "Even in rehearsal, Maria's fantastic performing ability had been obvious, and from then on, the others started trying ways of preventing her from appearing." Fellow singer Maria Alkeou similarly recalled that the established sopranos Nafsika Galanou and Anna (Zozó) Remmoundou "used to stand in the wings while was singing and make remarks about her, muttering, laughing, and point their fingers at her".

Despite these hostilities, Callas managed to continue and made her debut in a leading role in August 1942 as Tosca, going on to sing the role of Marta in Eugen d'Albert 's Tiefland at the Olympia Theatre. Callas' performance as Marta received glowing reviews. Critic Spanoudi declared Callas "an extremely dynamic artist possessing the rarest dramatic and musical gifts", and Vangelis Mangliveras evaluated Callas' performance for the weekly To Radiophonon:

The singer who took the part of Marta, that new star in the Greek firmament, with a matchless depth of feeling, gave a theatrical interpretation well up to the standard of a tragic actress. About her exceptional voice with its astonishing natural fluency, I do not wish to add anything to the words of Alexandra Lalaouni: 'Kalogeropoulou is one of those God-given talents that one can only marvel at.'

Following these performances, even Callas' detractors began to refer to her as "The God-Given". Some time later, watching Callas rehearse Beethoven 's Fidelio, rival soprano Anna Remoundou asked a colleague "Could it be that there is something divine and we haven't realized it?" Following Tiefland, Callas sang the role of Santuzza in Cavalleria rusticana again and followed it with O Protomastoras (Manolis Kalomiris) at the ancient Odeon of Herodes Atticus theatre at the foot of the Acropolis.

During August and September 1944, Callas performed the role of Leonore in a Greek language production of Fidelio, again at the Odeon of Herodes Atticus. German critic Friedrich Herzog, who witnessed the performances, declared Leonore Callas' "greatest triumph":

When Maria Kaloyeropoulou's Leonore let her soprano soar out radiantly in the untrammelled jubilation of the duet, she rose to the most sublime heights...Here she gave bud, blossom and fruit to that harmony of sound that also ennobled the art of the prima donna.

After the liberation of Greece, de Hidalgo advised Callas to establish herself in Italy. Callas proceeded to give a series of concerts around Greece, and then, against her teacher's advice, she returned to America to see her father and to further pursue her career. When she left Greece on September 14, 1945, two months short of her 22nd birthday, Callas had given 56 performances in seven operas and had appeared in around 20 recitals. Callas considered her Greek career as the foundation of her musical and dramatic upbringing, saying "When I got to the big career, there were no surprises for me."