从句意上来讲，which we cannot grasp这个定语从句修饰的并不是significance。再考虑习惯搭配invest sth. with sth.，这里的定语从句修饰的先行词是that，故这里的that是指示代词而不是关系代词。调整一下语序就是

We should not seek to invest that which we cannot grasp with significance .