Partly from the influence of fellow students, among whom was Franz Delitzsch, he adopted Christianity and was baptized with the middle name Paul in 1838.

His Jewish training naturally fitted him for work in Old Testament exegesis, and he spent two years at University of Berlin studying under German Lutheran neo-Lutheran theologian Ernst Wilhelm Hengstenberg.

The view found adherents in Denmark and Norway, and fear was felt that the formal principle of the Lutheran Church was in danger.

Caspari undertook a careful investigation of the questions connected with the baptismal formula, and its history and thus was led on to extensive ecclesiasticopatristic studies.

With Gisle Johnson he established in 1857 the Theologisk Tidskrift for den evangelisk-lutherske Kirke i Norge, of which a volume appeared annually until shortly before Caspari's death.