This traditional hymn is thought to have originated in India in the 19th Century among the Garo people, probably in what is now Meghalaya/Assam, and only later became a widely sung English languatge hymn.
The text is often connected to a Garo man named Nokseng, whose family was reportedly martyred after their conversion under American Baptist missionary work in Assam. According to this narrative, when ordered by a village chief to renounce Christ, he replied, “I have decided to follow Jesus,” then, after his children were killed, “Though no one joins me, still I will follow,” and finally, as his wife was killed and he himself faced death, “The cross before me, the world behind me.” This testimony is said to have moved the chief and other in the village to faith, and these last words were taken up as a song among Garo Christians.
Other sources (https://www.hymnologyarchive.com/i-have-decided-to-follow-jesus) attribute it to Simon Kara Marak, (1877 - 1975) in Kamrup, Assam, India. One of his daughters, Onima, who was born in 1938, testified, “From the time I started knowing things as a child, I heard my father singing this song. Therefore, I feel he wrote this song before I was born." A version printed in Christian Hymns, an Assamese hymnal, in 1960 includes these verses: