Cai Gao (1788–1818), also known as Tsae A-ko and by various other names, was the first Protestant convert in mainland China. He has also been called the first Western-style type-cutter and letterpress printer.

The real name of China's first Protestant convert is uncertain, although his surname was "almost certainly" 蔡. Like those of his family members, his name was recorded only in the missionaries' English romanizations, which include "Tsae-a-ko", "A-fo", and "A-no". Over the next two centuries, this was variously modernized as "Tsae A-ko", "A-Ko", "Ako", and "Ko". It has become generally accepted that these rendered the given names 蔡高 and 蔡亞高, which would be Cai Gao or Yagao in pinyin. Su and Ying, however, believe the original name to have been 蔡軻, which would be Cai Ke in pinyin. Smith also gives the Cantonese form as Choi A-ko.

Cai Gao's father was a Cantonese merchant at Macao whose legal wife had borne him no children; Gao's mother was his second concubine. He had an elder brother (born c. 1782) whose name variously appears as "Low Hëen", "Low-hëen", "A-hëen", and "A-këen". He had another brother "A-yun" or "Ayun", who was "a child", younger than Low-hëen and whom Morrison "wish[ed] to educate". Though frequently taken as Gao's younger brother, A-yun appears as his "elder brother" in the Mission Society's 1819 report. Morrison's journals mention an "A-Sam" who was distinct from A-yun and, as "a lad", distinct from his older tutor and companion Yong Sam-tak. McNeur, Gu, and Zetzsche do not mention A-yun as one of Gao's brothers but do say that "A-sam" was his younger brother.

The use of lou⁵ (老, p lǎo, "old") and aa³ (then 亞, p yà or yǎ, but now 阿, ā, "dear ~, dear little ~") reflects the standard practice of Cantonese nicknames. Morrison notes that these were generally used without "Tsae, being the sacred or family name". As the elder member of the family, "Low-hëen" received the affectionate honorific "low-" and the others the diminutive "a-". While it is possible "Low-hëen" and "A-hëen" were different members of the family, they are not distinguished as such by Morrison.

Cai Gao was born in 1788. He and his brothers were given a good Chinese education, although Gao's poor health caused him to fall behind his brother Low-hëen. His calligraphy, however, was quite good. During his adolescence, his father's wealth was lost when his ship returning from Batavia (now Jakarta) in the Dutch East Indies (now Indonesia) was wrecked in the South China Sea. The father died when Gao was sixteen. Upon the ruin of their family, their father's debts were so large that Gao's brother Low-hëen was suddenly arrested and imprisoned over a decade later for the unpaid amount.

In March 1808, Gao—then 20—began working for Robert Morrison at his home in Guangzhou's Thirteen Factories trading ghetto. Morrison was the first Protestant missionary to the Qing Empire and a translator for the East India Company. With the assistance of his Chinese staff, he published the first Chinese-language periodical and, with additional assistance from William Milne, wrote the first major Chinese–English, English–Chinese dictionary from 1815 to 1823. Low-hëen was already Morrison's tutor and companion, teaching him Cantonese and copying out Morrison's Chinese translation of the Bible. Gao was recommended by Yong Sam-tak, who had served as Morrison's comprador since February 1801, to be Morrison's printer, carving the wooden blocks necessary to publish the Chinese characters of his text. He also took charge of Morrison's shopping and provisioning. Morrison took note of him, saying "There is one boy, a fatherless lad, the brother of Low-heen. He possesses tolerable parts. I wish to pay attention to him."

Morrison asked his employees to attend Sunday worship services at his home. There were also daily meetings which began with Morrison praying, followed by a reading from his Chinese translation of the Bible and Morrison's commentary on it. They ended with hymns. These meetings were sometimes followed by personal counseling. At first Cai Gao was unable to understand Morrison's attempts to discuss Christianity, but he assisted Low-hëen in printing Morrison's Chinese translation of the New Testament. After about five months, he began praying with Morrison in Chinese. Nonetheless, owing to his quarreling with Morrison's Mandarin tutor "Kwei-Une", the missionary fired them both in late September 1808. Morrison never brought him back into personal service, even after his conversion, but hired him two years later as the printer for his missionary periodical. In his survey of the development of Western-style printing in China, Reed calls Cai Gao "the first Chinese type-cutter and letterpress operator".

Low-hëen continued to work with Morrison and Cai Gao continued to join him for Morrison's meetings and Sunday services. (Low-hëen was an ardent Confucianist, uninterested in Morrison's faith, but attended out of his sense of obligation to his employer.) By October 1812, Gao was reading Morrison's Bible before the group, expressing his own ideas about religion, and requesting Morrison's guidance on proper prayer. On October 30, he brought Morrison some idols, saying that he agreed with their uselessness. On November 8, he admitted that he desired baptism, but so secretly that his brothers wouldn't know. His bad temper and quarrelsome nature caused Morrison to deny his request. After a period of improved behavior and various testing, Cai Gao wrote out a statement of faith and Morrison relented. The confession follows the structure of a catechism, noting his own sins and "complete depravity", the need for salvation, Christ's ability to provide it and good works' inability, and his hope of resurrection.