As the son of a reformed pastor Kooiman began a study of theology in 1921 in Groningen. He became inspired by the historian of religion G. van der Leeuw (1890-1950), known particularly through his vast influence on the development of the liturgy.
In 1927, Kooiman, who by then had become a Lutheran, started as a pastor in the Lutheran congregation Wildervank-Veendam.
In 1935 Kooiman became the Lutheran pastor in Amsterdam and at the same time secretary of the committee responsible for the building of the Maarten Luther Kerk in Amsterdam-South.
In 1941 he published a study on the history of the Ronde Lutherse Kerk in Amsterdam. Two years later he received his doctoral degree in Amsterdam on ‘Luther’s hymn in the Netherlands ‘.
From 1946 until his death in 1968 he was professor at the University of Amsterdam and at the Evangelical-Lutheran Seminary.
Kooiman taught his students to listen to Luther critically. To him Luther was both a historic figure and a theologian who could easily bridge the gap between himself and later generations.
Kooiman received international recognition with his article on God’s hiddenness in Luther’s theology , with the distinctive title: ‘God’s play with masks in the theology of Luther’ (1955). Through this work he received an honorary doctorate degree in 1960 from the University of Münster. During the same time he served as president of the ‘Internationale Kongress für Lutherforschung’.
It was also important for Kooiman to make his research on Luther accessible to a broad public audience. His lectures for the radio were well known as was the book ‘Luther, his way and work’ that resulted.