It was not who is right but what is right,we all think,that

matter在这里是不及物动词 （vi.）可以理解为：要紧,有关系,所以不用were.句子中间的we all think 是插入语,句子主体是强调句：It was not who is right but what is right that matters.
matter 例句:
It does not matter at all.
毫无关系.
It doesn't matter throwing that away.
把它扔掉,没关系.
It never mattered what time of day or night it was.
不论是白天还是晚上,都没有关系.
It doesn't matter much whether we go together or separately.
我们一起去还是分头去都可以.

It is ... that... 是强调句的形式，意思是正是。。。才。。。
你可以去掉这个句式来看，以及插入语we all think，这样就变成:
Not who is right but what is right matters.这样句子的结构就十分清楚了
因为是分开的两个从句之间是转折关系，所以不用were。
希望能帮助到你，祝你学习顺利，请采纳吧...

这一题考察的依然是一个强调句，not who is right but what is right就It is that这个句型 it is .that强调句型 去掉之后句子结构依然

此句译为：最为要紧的不是我们都认为的谁是对的而是什么是对的。that matters 为强调句的后半部分。