Complex sentence: Johnston first came to public notice in 1637, during Charles I's attempt to force an Episcopalian liturgy upon Scotland, when as the chief adviser of the covenanting leaders he drew up their remonstrances.
Split into simpler sentences: Johnston first came to public notice in 1637 during Charles I's attempt to force an Episcopalian liturgy upon Scotland. As the chief adviser of the covenanting leaders Johnston drew up their remonstrances.