All the saints attained salvation not only by prayer, but also(they attained salvation省略) by fasting.
备注(1):but also 是省略主语+谓语动词+宾语 的一个分句。they指代all the saints
备注(2):逗号可以加,也可以不加,如果强调可以加逗号,反之不加逗号
He not only read the book, but also remembered what he had read.【有逗号】
他不但读了这本书,而且记得所读的内容。
She not only wrote the text but also selected the illustrations.【无逗号】
她不仅写了正文部分,而且还挑选了插图。