Affirming The Consequent?

As presented, I think you are correct it is affirming the consequent. Which is when the antecedent in an indicative conditional is claimed to be true because the consequent is true; if A, then B; B, therefore A. It is an argument of the form m→n; n, therefore m. A common example is: “If you are drunk, then your cognitive abilities are impaired. Your cognitive abilities are impaired. Therefore, you are drunk.

However, on a practical level, I see two significant hurdles to overcome before looking at the proposition. The participants must agree on the definition of “god” and “evil.”

Nonetheless, this appears to me to be a partial restatement of the problem of evil.  Epicurus expressed this problem thus: Either god would remove evil out of this world and cannot, or he can and will not, or he has not the power nor will, or, lastly, he has both the power and the will. 

If he [god] has the will and not the power, this shows weakness—which is contrary to the nature of god. If he has the power and not the will, it is malignity, and this is no less contrary to his nature. And if he is neither able nor willing, he is both impotent and malignant, and, consequently, cannot be god. And if he is both willing and able, which alone is consonant with the nature of god, whence comes evil? Why does he not prevent it?

Hume, and others, have worked on this problem. 

I agree with the affirming the consequent. 
1)If God exists, then good and evil exists.
2)Good and evil exists
3)Therefore God exists.

I would like to comment though on the ridiculousness of such "arguments". It is as valid as the following argument:
1)If Superman (one could also choose Batman) exists, then good and evil exists.
2)Good and evil exists.
3)Therefore Superman exists.

Also, ascribing evil actions to the devil is a case of scapegoating and saying that good is God (or evil is Devil) is a case of reification . I think it is also fantasy projection to mix real (human) actions with mythical creatures. And as all religious claims, it is also a case of unfalsifiability  

[Updated]

As others have pointed out, this is not the fallacy you suggested. There do not seem to be any obvious fallacies herein. However, an argument can be logically valid and also be built on false premises (as in an "argument from false premises"). I see several premises that are suspect:

1. The first statement assumes evil is dependent on the existence of God.


2. The third statement assumes evil should be defined as "the absence of good."


3. The fourth statement assumes you can have evil without good.


4. The fifth statement assumes that God exists (in order to later conclude that God exists).


5. It also assumes God is the ultimate good.

As as result, the conclusion is perched atop a foundation that is a house of cards, the cards being assumptions the arguer is asking you to accept. Its simpler form is better:

1. If God does not exist, then evil does not exist.

2. Evil does exist.

3. Therefore, God exists.

As has been pointed out, this is a valid syllogism. Yet the first statement is still begging a big question (although not begging THE question): the question of whether God and evil are dependent on each other. It is a questionable premise at best, a false one at worst.

If I can assume some context here, the arguer is an apologist attempting to respond to a common atheist challenge, what apologists call "the problem of evil." As demonstrated above, this cannot be done with syllogisms. It can be done, but not in this way.

Finally, said atheists also often make simple logical errors when tackling this topic. In my experience, they get trapped in self-defeating arguments. The apologist doesn't have to sneak in assumptions because the atheist goes ahead and does it himself, not realizing he is creating what apologist Greg Koukl calls "arguments that commit suicide." Here's an example:

Atheist: Your god is a fantasy. The Bible is a collection of fairytales. You may as well believe in Zeus.

Apologist: God is not at all like Zeus.

Atheist: Yeah, instead he's a genocidal maniac who committed all these horrible atrocities, such as [cites various misconstrued events from the Old Testament].

Apologist: So your argument is someone you don't believe exists committed atrocities according to your interpretation of events you believe didn't happen?

The bottom line: In order to tackle the question, "Why does God allow evil?" you must first accept the existence of God and the existence of the Judeo-Christian conception of evil, which is rooted in the Bible. In other words, this is not a good topic for atheists.

Hi, Jack!

         Not so fast. The argument you have posted, Jack, is a valid argument and does not commit the affirming the consequent fallacy. The first premise states that if God does not exist, then evil does not exist. But look at the second premise. The second premise states that evil does exist. The proposition that evil does exist is the denial of the consequent that evil does not exist. The argument has a Modus Tollens form. The argument validly infers that God exists. Formally,
 
If not G, then not E
E
Therefore, G
 
         The argument would have been affirming the consequent if the second premise stated that evil does not exist, and if the conclusion was that God does not exist. Formally,
 
If not G, then not E
Not E
Therefore, not G

What makes this tricky is (1) the definitional premises inserted into the argument (these do nothing for the form of the argument, so they can be removed) and (2) the negation of P is used in the conclusion.

If God does not exist (P), then evil does not exist (Q).
Evil does exist (!Q). 
Therefore, God exists (!P).

Removing the bias...

If I don't die, then I will not be buried.
I was buried.
Therefore, I died.

This is not the affirming the consequent due to the negation of Q in the second premise and P. In the first premise, we are setting the condition that in order to be buried we MUST die. The rest of the argument confirms that.