In the 1920s, after the failure of attempted Bolshevik revolutions in Europe, the Commintern took a long and detailed look at what had gone wrong.
After all, Marx had predicted that revolution would happen as a reaction to the most advanced stage of capitalism, so the revolution should have been in countries like Germany, Britain and the USA, not in Russia.
The study concluded that the workers of the West were able to get reforms through trade unions and so did not see the need for revolution. So the Communists decided to switch their efforts away from the workers and attempting revolution from below.
Instead they would concentrate on recruiting young people destined to go on to become the movers and shakers of Western societies. So they set to infiltrating the teacher training colleges, the top universities, journalism and the churches and seminaries.