天经地义的英文：
[Literal Meaning]
heaven/principle/earth/reason
both natural and right

[解释]
经：规范，原则；义：正理。天地间历久不变的常道。指绝对正确，不能改变的道理。也指理所当然的事。

[Explanation]
universally accepted principle

[例子]
不要以为世界上有什么天经地义的事情，一切都会改变的。

[Example]
You should not think that there is anything that is regarded as unalterable principle in the world. Everything changes.

[英文等价词]
God's truth

Since customers are regarded as the god, it will be an unalterable principle to foster good relations with them.
既然客户被称做是上帝，那么和客户搞好关系就是天经地义的事情啦。