the phrase seems as taken from the Bible translation to me therefore please remember one important thing about Chinese language: "trying to get the meaning of a Chinese phrase -- make sure it's a _correct_ Chinese phrase" so 因此 ， 犹太人 问 他 说 ，你 既 作 这 些 事 ，还 显 什么 神迹 给 我们 看 呢 ？ = Thus, the Jew asked him, as you have already done this, what else / other miracle do you want us to see / to show to us. 既 means "already", makes it "have done" instead of "did"

“既。。又。。”和“既。。还。。”基本一样 但是你的句子“因此 ，犹太人问他说 ，你既作这些事 ，还显什么神迹给我们看呢 ？”不是很地道。。应该用“既然”

In that sentence, 既 means 既然 (= since), so 既....还 is NOT a set pattern. It has a completely different meaning from the set pattern "既...又". We rarely use 既 to mean "既然". So actually "你既作这些事 ，还显什么神迹给我们看呢 ？" doesn't sound very natural to me.

既........还........ 表示说话意思的转折，比如说：既然你知道还问我？ 既........又........ 表示说话意思前后并列，比如说：他既是老师又是医生。

神迹在这里多指本事/本领. 我想作者的意思应该是:你不是觉得你很有本事吗?既然你已经做了,还在这儿显摆给我们看做什么?