my brother who doesn't pray has photographic memory, he can memorise texts and pages. So what do you mean "weaken their memory"?
JazakAllah khayran.
And Allah :swt: knows best.
The `Ulamaa have explained that when a person sins against Allaah Ta`aalaa, that sin has a negative effect on the body of the person as well as his mind. His body becomes slightly weaker, and his mind (including his memory) becomes slightly weaker. Some people have been granted extremely powerful memories, thus they are able to memorise easily. Sin will adversely affect even their powerful memories, although they may still be able to memorise entire documents, due to the fact that Allaah Ta`aalaa has naturally granted them a much stronger memory than other people. Similarly, you get many people who sin, yet they are still seen to be physically strong. This is because the sins are affecting their body, but, because they are relatively stronger than other people, it is not noticeable yet. Were they to not sin, their bodies would be stronger. Were a person to not sin, his memory would be stronger.
Let me give an example.
According to medical professionals, reading books (hardcover and especially e-books) weakens a person's eyesight. You've heard of this? Eventually a person has to start wearing glasses because the reading has weakened his eyes, especially if he's been spending hours everyday reading on a computer screen. Correct? Yet, I've read thousands of books in my life, and almost a thousand of them have been on computer, yet my eyesight is still almost perfect and I've never needed glasses in my life, Alhamdulillaah. Once, I read 2,100 pages on computer in a single week. Yet I have better eyesight than many people that I know of, and they need to wear glasses despite having read much less.
So there is the rule, and then you get those people who are the exception to the rule.
Take another example.
Medical professionals say that children should not be given too many sweets, chocolates, etc. because it will ruin their teeth. Correct? As a child, I ate at least 50x more sweets, chocolates, chips, soft drinks, etc. than anyone I know of. As a five year old I used to drink an entire two-litre cola in a single day. So you have an idea what I'm talking about. Yet, all my teeth are in perfect condition and in my entire life I've never needed to go to the dentist. Ever. I've never been to the dentist as a patient. Not even once, in all my years. Alhamdulillaah.
Do you see what I mean? How will the medical professionals answer that? They can prove medically that sweets and things really do ruin your teeth, yet they didn't ruin mine. Why? Allaah Ta`aalaa safeguarded me. I was an "exception to the rule".
So again: The rule is that when a person sins against Allaah Ta`aalaa, their body and mind weakens. Yet, some people have been granted very powerful bodies and very powerful minds by Allaah Ta`aalaa, so when they sin, you might not notice the adverse effects those sins are having.
I hope the matter is clear, In Shaa Allaah.
والسلام