With Apple (NASDAQ:AAPL) stock down about 22% at the time of this writing from its all-time high of $134.54 set earlier this year, and considering how cheap the shares are currently trading relative to underlying fundamentals, it's a great time to review some of the key reasons the tech giant could continue to see robust growth in its business going forward. On this note, here's a close look at arguably Apple's most-important catalyst of all: China.

As Apple CEO Tim Cook recently emphasized in a public letter to CNBC-TV show host and investor Jim Cramer, the company's opportunity in China remains robust. Indeed, Cook went as far as to refer to the opportunity as "unprecedented." But for investors who want more specific reasons to be bullish about this catalyst, there's more tangible data to latch onto.

Here are two key ways to view China as a catalyst for Apple.

1. Domination at the high-end. A 2014 study by Chinese analytics firm Umeng found that, among the $500-plus smartphones in China, which accounted for 27% of total smartphones in the market, a whopping 80% were iPhones. And with Apple's year-over-year smartphone growth since then handily outpacing the overall market growth, it's likely this figure is even higher today. The sentiment toward Apple products in the country is, therefore, incredibly positive.

Apple has benefited tremendously from this favor toward its iPhones in China with the launch of the iPhone 6 and 6 Plus. During the two full quarters these iPhones have been available in China, fiscal Q2 and Q3, revenue for Apple's Greater China segment, which includes China, Hong Kong, and Taiwan, soared 71% and 112%, respectively.

2. China Mobile is still a growth driver for Apple. Headed into 2014, there was plenty of buzz about the looming growth opportunity for smartphone manufacturers, as the world's largest wireless carrier, China Mobile, prepped to launch its LTE network. The network would spur higher demand for bandwidth-hogging, high-end smartphones like Apple's iPhone, proponents argued. Since China Mobile's LTE launch, which was well timed to occur in conjunction with the launch of the first iPhone on the network, this has been exactly what has happened -- and Apple has been the primary beneficiary.

Looking back, the growth of China Mobile's LTE network is simply astounding.

Data for chart retrieved from China Mobile's monthly subscriber counts. Chart source: Author.

During the last 12 months, China Mobile's 4G customers have ballooned from just 20.4 million to 208.8 million. And growth in China Mobile's LTE customers continues at a rapid pace, even in recent months. In the last two months, China Mobile's 4G subscribers increased by about 39 million.

This growth in the world's largest carrier's 4G customers has been a critical driver for iPhone sales. But don't dismiss China Mobile as Apple's glory days of the past. The company still has a 210-million person addressable market in the carrier.

Data for chart retrieved from China Mobile's monthly subscriber counts. Chart source: Author.

This huge opportunity for Apple in China Mobile to attract a portion of these 3G users as first-time iPhone buyers is representative of a marketwide opportunity in the world's largest smartphone market. As China's middle class continues to grow, and as wireless subscribers are buying 4G devices in droves, Apple is positioned to continue to benefit from rising smartphone sales in the market.

Investors shouldn't ignore this catalyst for Apple stock. As Apple's second-largest operating segment, and representing 27% of its total revenue, meaningful growth in Greater China can have a large positive impact on the tech giant's overall business.