Venice at that time was the capital of the republic of Venice.
The president of the union joined by his father, Giovanni Legrenzi, was maestro di cappella at the Basilica of San Marco and noted early Baroque composer. He was believed to have taught the young Antonio to compose. Scholar Walter Kolneder of Luxemburg figures that the liturgical work Laetatus sum ((RV Anh 31, written in 1691) written when he was just 13, showed influence of Legrenzi style. He began studying priesthood and in 1703, at the age of 25, he was an ordained Venetian priest and was nicknamed "Il Prete Roso" or The Red Priest but he did not practiced his vocation due to a sickness which is his tightening of chest, suspected as asthma. In 1704, he was given dispensation from celebrating the Holy Mass due to his illness and he withdrew from active priesthood in 1706.
By December 1703, Vivaldi was appointed maestro de violino at the Ospedale della Pieta, one of Venetian girls' orphanage. At that time, the institution had four branches in Venice giving shelter and education to orphaned, abandoned or children from impoverished families as funded by the Republic. The boys were taught a trade before leaving at age 15 while the girls had musical education and the talented were retained to be members of the Ospedale's orchestra and choir.
Ospedale under the ministration of Antonio Vivaldi began to gain appreciation even abroad as Vivaldi wrote most of the cantatas, concerto and sacred music. The position of teacher of viola all'inglese was also added on his duty as maestro de violini by 1704 and although he is already starting to prove his talent at that time, he was retained at the Pieta until 1709 only when the board dismissed him, for many perceived him a difficult man. He worked as a freelance musician for a year until the board recalled him in 1711 realising his potential.
From then, Vivaldi was in his most productive years writing music or operas and concertos. His first collection of his works called Raccolta, a trio sonata was published in 1705 with his Opus 1 a collection of 12 sonatas for two violins and basso continuo in conventional style. His Opus 2 is a collection of 12 sonatas for violins and basso continuo and was published in 1709. L'Estro Armonico (Opus 3) his first collection of 12 concerti for one, two, and four violins with strings was published by Estienne Roger in Amsterdam in 1711. It became a success all over Europe as followed by another success La Stravaganza (Opus 4) in 1714 which is a collection of concerti for solo violin and strings. Musicians sought sought him out in Venice and commissioned works from him including for the Dresden court.
Johann Sebastian Bach transcribed five Opus 3 concertos for keyboard and he influenced many German composers.
Antonio and his father went to Brescia in 1917. His Stabat Mater (RV 621) was played as part of a religious festival and although the work was apparently done in haste, the forced essentiality of the music revealed musical and emotional depth that made it one of his masterpieces. He also started traveling in 1718 to promote his music and in even during his travels, he retained connection with the Pieta and wrote two concertos each month sending them by post. He also found time to rehearse