The son of a Dutch publisher and his Scottish wife, Kruyt studied theology at Utrecht and joined the League of Christian Socialists in 1910.

[2] He formed an alliance with Willem van Ravesteyn and David Wijnkoop, the two representatives from the Social Democratic Party as well as Harm Kolthek.

When, in 1921, the League had disintegrated due to factional struggles between orthodox Protestants and anarchists, Kruyt joined the Communist Party.

[3] Following his wife's death, Kruyt emigrated to Berlin where he joined Workers International Relief, then to Moscow in 1935 (presumably fleeing from the Nazis)[4] to work at the Lenin State Library.

Kruyt was tortured and deported to the Moabit prison in Berlin where he is thought to have been executed by firing squad in July 1943.