"The Immortal Way is indistinct and the human way is vast" comes from "The Immortal Way Sutra", which is also from the regular script "The Book of Saving People" and the "Lingbao Wumei Measuring Man's High-grade Miaojing".
The author is unknown. The full name is "Tai Shang Cave Xuan Ling Bao Immeasurable Man's Excellent Grade Miao Jing", also known as "Yuanshi Immeasurable Man's Excellent Grade Miao Jing".
It is a classic of the Lingbao sect of Taoism. Sixty-one volumes. The first volume is the text of "The Sutra of Saving People", which is one of the ancient spiritual treasure sutras of the Eastern Jin and Southern Dynasties. They all state the ultimate truth and are the first of the three sutras. The theme of the full text is "The Immortal Way cherishes life, but there is no measure of human beings." Promoting fasting and chanting sutras has great merits. It can eliminate natural disasters and protect the emperor, and can save the people from poison. Men and women are protected and can live forever.
Full text and translation:
Humanity is vague, immortality is vast, and ghosts are happy! When one enters the human life, life is precious in the immortal way, and death is precious in the ghost way; the immortal way always brings good luck, and the ghost way always brings misfortune; the high, pure, spiritual and beautiful, the tragic song is loud in the sky; I only wish that the immortal way can be achieved, and I don’t want to be poor in the human world.
There are thousands of ghosts in Quanmiao Mansion, the northern capital. But if you want to suppress people's calculations and cut off people's life gates, Ah Ren sings the cave chapter to capture Bei Luofeng, chants the demon spirits in bunches, and kills the six ghost fronts. The weather is turbulent, and my Tao is prosperous.
Translation:
The way of life is vague and boundless and cannot be seen clearly, the way of cultivating immortals is confusing and far away and difficult to grasp, but the way of ghosts can indeed bring temporary happiness to people! But people should follow the path of peace and harmony. The way of cultivating immortals is to value life and value life. The way of ghosts is the end of preferences. The path of cultivating immortals often makes people naturally auspicious, while the path of ghosts often makes people dangerous and evil. Therefore, the spirit of tranquility and inaction regards beauty as the highest priority, and one must not lament in lamentation that all things are empty. I only want to achieve great success in cultivating the way of heaven, and I don’t want the road of life to end.
Fengdu City, Quanmiao Prefecture in Beidu, is home to thousands of ghosts and ghosts. They want to curb fortune-telling and cut off the world's life gate. Who can sing the cave chapter, capture the ghosts of Fengdu in Beiluo, and capture the demon spirits? At that time, there will be great power in the heavens, and our Taoism will become more and more prosperous.
Reference for the above content: Baidu Encyclopedia-Xian Dao Jing