Training decreases the likelihood of you dying or being injured.
I'd tweak that and say experience coupled with reflection on the experience decreases the chances of you making a mistake (or a series of them) that results in death/injury. Training is a much safer way of gaining that experience than the earliest divers did: read up, suit up, and go see what happens. But the idea that your diving is necessarily crossing some magical danger threshold if you're not taking a specific formal course and receiving a particular set of c-cards before doing X dive is, frankly, bull
.
Training, especially the most useful training in the most demanding areas of diving, does not eliminate the risks that come with gaining experience. People die during training, and you can too. Any of us can die on any dive, regardless of training, c-cards, or experience, and while quantifying the difference in risk between the two ends of the spectrum (totally new diver without any of the right gear or training; fit, experienced diver with all the right gear and training) is easy enough it's still far from objective. Quantifying the difference in risk between a newly certified tech diver and someone who's read everything they could get their hands on, invested in the right gear and learned how to use it, and gained experience by gradually doing simpler versions of the more advanced dives they're interested in... yeah, good luck with that distinction. There are many non-obvious things one needs to know cold, and many techniques that need to be second nature, in order to do certain dives while being only as at-risk as possible in such an inherently life-threatening situation. How people pick those up is up to them, IMO, but at the very least you have to be constantly aware of the consequences of what you're doing.
Every time
any of us go under the water, we might not come back up (maybe the gear and corpse will)... I think people need to really believe and understand that and then do whatever they need to do in their own mind to protect themselves. For some, that means nothing more than slapping on an 80 while for others it means never diving at all. Everyone else is somewhere in between, and I find the haughty 'I'm special and on one side of the risk line because of my [c-card; instructor; agency; specific course; number of dives]' attitude to be amusing at best.
[EDIT] I see this is inexplicably in the Basic Scuba Discussions section, so I want to clarify for anyone reading this and thinking it's a justification for going your own way and ignoring all the people warning you off:
I don't care what happens to you because of your choices.
My most cherished belief is that respecting others means letting them make their own mistakes, no matter the consequences. Everyone else in here is, as far as I can tell and excepting a few holier than thou sorts, warning you because they actually care about what happens to you. Just something to keep in mind.
[/EDIT]