Through his Hebrew teacher, the mathematician and Hebraist Moses Lemans, he became acquainted with the great Dutch poet Willem Bilderdijk, who, at the request of Isaäc's father, agreed to supervise the boy's further education.

He was a faithful adherent of the religious views of his friend, was one of the leaders of the Orthodox Reformed party, and during the last years of his life was a teacher and a director of the seminary of the Independent Scotch Church.

Although he wrote much on missionary matters, he is distinguished from many other converts in that, to the end of his life, he felt only reverence and love for his former coreligionists, was deeply interested in their history, and often took their part.

He also wrote "Israel en de Volken" (2d ed., Haarlem, 1848–49), a survey of the history of the Jews to the nineteenth century, written from the standpoint of the Church.

The work was translated into English, under the title "Israel and the Gentiles," by Ward Kennedy (London, 1850), and into German by "A Friend of God's Word" (Miss Thumb), published by K. Mann (Frankfort-on-the-Main, 1855).

Da Costa possessed a valuable library which contained a large number of Spanish, Portuguese, and Hebrew manuscripts, as well as rare prints from the Spanish-Portuguese Jewish literature.