"The happiness which he attributes to religion he could have obtained from becoming a crossing-sweeper, provided he were compelled to remain one. " The happiness which he attributes to religion he could have obtained from becoming a crossing-sweeper if he were obliged to continue being a crossing-sweeper. the word "one" refers to a crossing-sweeper. So, the sentence means that he could acheive the same level of happiness in two ways 1- through religion 2- through becoming a crossing-sweeper and being obliged to remain a crossing-sweeper I hope that helps

"Provided he were compelled to remain one". I think it means "as long as he is dedicated to being a crossing-sweeper". 主要是他坚持，努力做这一份工作。