The very act of...

[Hektor6766]

Which of these would be better to say "The very act of stealing makes him a thief"?

彼はその盗難事で盗賊なる。

盗むという行為は彼泥棒になります。

Or what would be the best way?

[Hyperworm]

Of your two suggestions, the second one seems to have the best groundwork, but it's not quite there.
盗むという行為自体が彼を「泥棒」にしている。
盗んでいるという事実は彼を「泥棒」だと言えるのに十分だ。
maybe?

Not confident here, defer to native speakers...

[writebook]

盗むという行為自体が彼を「泥棒」にしている。
盗んでいるという事実は彼を「泥棒」だと言えるのに十分だ。

You've found good ways to translate "the very act of."
Just one small thing - "だと言えるのに十分だ" should be "だと言うのに十分だ".

This sentence won't sound natural if it is translated word-for-word into Japanese, because of two things:
1)The way something abstract or inanimate is being used as the subject.
Sentences like "His reply made me angry" sound kind of weird in Japanese, you have to rephrase it like "I got angry because of his reply."

2)The way the word "makes" is being used. It means here that he fits the definition of thief. In Japanese, you can use the word "make" to mean that it caused him to become a thief, the way you can say "Poverty made him a thief" or "His parents ordered him to steal and taught him how. They made him a thief." However, in Japanese you don't use the word "make" to mean "fit the definition of ---" (in sentences such as "You just saved somebody. That makes you a hero.")

You would have to choose a new approach to say the same thing. I would say it like:
ものを盗んだのなら、その人はもう泥棒だ。
If he has stolen something, then he already is a thief.

ものを盗んだのなら、その人はもう泥棒だといえる。
If he has stolen something, then it can be said that he already is a thief.

[Hektor6766]

I found an amusing example on Space ALC; fairly close, no inanimate actors:

われこそ将軍にふさわしい人物じゃ。わっはっは。

Roughly,

"I'm the very model of a shogun. Wa-ha-ha!"