"Where does evil come from" is a simple question, and an obvious question!
Christians ought to be able to answer the question, in the same simple and obvious terms it is stated - without recourse to vast, complex and soaring abstractions - and would-be-profound discourse on the nature of 'infinities'!
And answered without recourse to statements that "Men cannot know", such things - or that they are divine-mysteries...
Why? Because the question is obvious, reasonable, real, serious; and therefore will (deservedly!) block conversion unless answerable in the same terms it has been asked.
The basis of the question is that there are obviously evils in this world; and Christians also know that God - the creator of this world - is Good: So where do these evils comes from?
The proper answer - the answer demanded and deserved - should be short, concrete and to the point; or else it is not a real answer
...and will be recognized by most people as not a real answer.
If God is supposed to have created everything except Himself (i.e. if God is an Omni-God) and is also supposed be wholly Good (i.e. if the real Christian God was also the Omni-God) - then where does evil come from?
It's a valid question. And there is no coherent answer: If God is Omni then he cannot be wholly Good, and if wholly Good then he cannot be Omni.
And this incompatibility has always been obvious to most people in the world; including, I would guess, many Christians.
One attempt at arguing the compatibility of Omni-God and Christianity is to insist that evil and Good are actually, ultimately, the same and indistinguishable aspects of God's will - that both apparent-Good and apparent-evil come from God; and it is only our erring, limited human minds who cannot see this oneness.
Indeed, taken one at a time - any proposed evil can be reframed as potentially an aspect of Good - in some larger sense.
However, even though individual instances of putative evil can be put-in-doubt, to insist on the oneness, inseparability of Good and evil-as-such is Not to be a Christian. If there is no evil, no 'sin' - if all is an equal expression of oneness - then there is no point to Christianity.
So that kind of argument does not work.
If evil is really-real, and God is really Omni - then evil must come from God, evil must be located-in God, ultimately.
The usual Omni-God answer is that God gave some creatures - including Men - agency, or free will; so that Men can choose to oppose God and creation.
Okay - but where does that motivation to oppose God and creation come from?
If every Man was wholly-made by God (as the Omni-God concept insists); then - even if agency-within-omnipotence was granted - where does the motivation to evil come-from? It has to come from God, ultimately, because (with an Omni-God) there is nowhere else for motivation to come-from.
In more general terms, all evil motivations, indeed the whole possibility-of-evil, would need to be baked-into the created world by the Omni-God - because the Omni-God ultimately created every-thing.
Evil is best understand as opposition to God and to God's creation... So how can there be opposition to God and creation from some being who is created by God?
One answer is to regard evil as a kind of insanity: something that doesn't make sense.
For instance, Satan is described in terms of knowing that he was created-by God, but (for some crazy reason!) hating and resenting this fact; and therefore hating himself - along with the whole of creation.
Okay - but where did such insanity originate - if not the Omni-God Himself?
How was it possible that one of God's creatures could develop this kind of pathology of insane self-hatred, resentment - pride? The only ultimate answer is that God-made-him-that-way - because, with Omni-God, that is eventually the answer to all questions...
For Omni-Godites; God made Satan that way - for reasons we cannot comprehend, but which God comprehends - therefore, the duty of all good Christians is submission, obedience to God's will...
...But wait a minute! Uncomprehending submission to God's incomprehensible will... That sounds very much like another religion; but it is not Christianity - it is not the kind of thing that Christ would teach in the Gospels, and of course there is no essential role for agency.
And this not Christianity; because this argument means that Christians are regarded as incapable of personally recognizing and rejecting evil...
The Omni-God answer to any version of the question: "Where does evil come from?" can only either be God - or else, the denial that evil really is evil.
And either or both of these lead either to a God beyond human discernment, a God to whom we must submit without understanding; or else the denial that evil really is different-from Good.
Both of which are incompatible with Christianity.
The incompatibility of the Omni-God with Christianity is indeed very easy to see and impossible to deny without redefining Christianity; or else moving to a level of uncomprehended-abstraction piled-upon uncomprehended-abstraction that makes coherent thinking impossible - and seems intended to stun all opposition into silence!
And this is why all honest defenses of a Christian Omni-God always end-up with putting 'incomprehensible mystery' at the very heart of the Christian religion - and the Christian is required simply to accept that.
Which is hardly a satisfactory state of affairs. Especially not for someone considering conversion to Christianity, or to a Christian who wants a proper answer to an urgent question.
So if the question is "where does evil come from?" - then what would I personally regard as an answer that is short, concrete and to the point - and true?
My answer is that:
God the creator is indeed wholly Good, however God creates using already-existing Beings - such as Men. It is these already-existing Beings that are the source of evil.
In other words, God's creation includes evil because it is made from beings that were already-evil; in different ways, and to different degrees.
Or, God is wholly Good, but Men are - and always have been - partly evil.
Therefore; evil really is evil; and God the creator really is Good - but the Christian God is Not an Omni-God.
Now - of course! - such an answer raises further questions; and invites expansion.
So much for this mortal earth; but can we not look-forward to inhabiting a wholly-Good world? What about Heaven the wholly-Good place, for example?
How can Heaven be really-real and wholly Good - when the beings in God's creation contain evil?
It's another question; and it needs another answer.
A short answer to this question is: that is where Jesus Christ comes in.
This world contains evil because men brought evil into it; but for those who follow Jesus Christ, the next world can be wholly Good...
Because after death, and through resurrection, Men can choose to leave all evil behind and take-forward into Heaven only that which is Good.
Note: Whether or not you regard my short, simple and to-the-point answers as sufficiently correct (given that all possible answers are bound to be incomplete and biased, and especially short answers!) - I hope they at least illustrate the kind-of-answer that is required to this question. And that the above suggested answers may demonstrate, by contrast, the kind-of-answer that is not suitable to the question.
Further note: Those seriously interested in agency, which itself is the other side of the coin of evil - and in the Omni-God (aka Supergod) in relation to Christianity - should read William James Tychonievich's superb essay, published today.