Here’s the deal. Just like Abraham “committed to God, and that commitment was what made him right,” so in the same way everyone who commits to God are Abraham’s children. As far as scripture is concerned, heathen come to God the same way. It even says, “All the heathen will be blessed in you, Abraham.” So anyone who commits gets the same blessing Abraham got by committing.
There’s no such thing, if you take Paul seriously, as evangelism by procreation. You are not, spiritually speaking, what your parents were. You make your own commitments.
In it’s original context, it means you didn’t have to be Jewish to follow the Jewish God Jesus believed in. That was pretty radical in it’s time.
Now it also means that you don’t have to belong to any religious family just because you were born there. It means that you can be anything you want to be. You have to make your own commitments.
Of course, if you have to make your own commitments, you have to put some effort in. You can’t just cruise along through life saying, “I’m a Presbyterian,” (or whatever you say you are) without really committing to it. An uncommitted Presbyterian (or Methodist, or Baptist, or Catholic) isn’t really a Presbyterian (or Methodist, or Baptist, or Catholic) at all. I’m pretty sure there are a lot of atheists who are as uncommitted about their atheism as many religious people are about their religion, too. And it probably works for Buddhists, Muslims, and Hindus.
The point is that with religion, like with anything else (mathematics, art, business), there is no benefit without commitment.