Complex sentence: From his earliest years he had been deeply impressed with the piety, beauty and thoughtfulness of the writings of the Christian mystics, but it was not till after his accidental meeting with the works of Boehme, about 1734, that pronounced mysticism appeared in his works.
Split into simpler sentences: From his earliest years, he had been deeply impressed with the piety, beauty and thoughtfulness of the writings of the Christian mystics. However, it was not till after his accidental meeting with the works of Boehme, about 1734, that pronounced mysticism appeared in his works.