He then returned to Poland in 1823 to lead the mission to the Jews in Warsaw and served as the English chaplain until 1830, despite conflicts with Lutheran congregations.
In Berlin, he gained the support of the English ambassador George Henry Rose and the Crown Prince of Prussia.
He played a significant role in the Convocation of the English clergy, where he initially opposed, but later collaborated with, the High Church party.
He was for some time tutor to the Earl of Rosse, and then, was sent in 1821 to Poland as a missionary, by the London Society for Promoting Christianity among the Jews.
Moving to Berlin, where he was befriended by George Henry Rose, the English ambassador, and by the Crown Prince of Prussia, who had known him at Warsaw.
[2] In 1840, McCaul was appointed principal of the Hebrew college founded by the London Society; and in the summer of 1841, through Frederick William IV of Prussia, he was offered the bishopric of Jerusalem, but declined it because he thought it would be better held by one who had been a Jew.
In 1844[3] he became prebendary of St Paul's Cathedral, and in 1847 declined Archbishop William Howley's offer of one of the new colonial bishoprics.
When the sittings of Convocation were revived in 1852, McCaul was elected proctor for the London clergy, and represented them for the rest of his life.