What is the use of "的" in "辞的职"?
辞职: resign; quit the job;
辞： quit; take leave; resign; dismiss;
职（in 辞职）is short for 职位(not 职业)： position；
So, 辞职 can be interpreted as: quit your position or just resign.
辞职 is a 离合词，so it's separable. Phrases like 辞的职, 吃的饭, 说的话, and etc denote that you have done something at sometime or at some place and usually they are followed or preceded by a prepositional phrase describing time or position.
In your example,  来中国之前(before coming to China) is the time and 辞的职 is what you have done(quit your job). So, 来中国之前辞的职 is quit my job before coming to China.
More examples:

我在家里吃的饭 // I ate my meal at home.
这是我昨天在会议上讲的话 // This is what I said at the meeting yesterday.