一乘，又称‘一佛乘’。系与‘三乘’相对的教法。三乘教法谓众生在修习佛法时，有声闻、缘觉、菩萨等三种差别，而一乘教法则谓小乘之声闻、缘觉教法到最后亦终将与菩萨乘的行者相同，都将成佛。因为声闻乘与缘觉乘的教法只是一种权巧方便，并不是佛陀的本怀。佛陀为一大事因缘出世的目的，是在引导一切众生终皆成佛。因此，以成佛为最终归趋的一乘 [P10-a] 教法才是佛陀弘法的真正意趣所在。这种主张是《法华经》最主要的特色所在。《法华经》〈髐隢K品 〉曾谓（大正9?7b）︰‘诸佛以方便力，于一佛乘分别说三。（中略）十方佛土中，唯有一乘法，无二亦无三。’并且以羊车、鹿车、牛车比喻三乘，而以大白牛车比喻一乘。可见该经的主张，显然以为三乘法仅是方便法门，唯有一乘法才是真实之教。

“一乘”对应的英文表达为“Ekayāna”，也译作“one vehicle”。

Ekayāna (Sanskrit: एकयान, traditional Chinese: 一乘; pinyin: Yīchéng; Japanese: いちじょう; Korean: 일승) is a Sanskrit word that can mean "one path" or "one vehicle". It is used both in the Upanishads and the Mahāyāna sūtras.

one vehicle ［一乗］ ( ekayāna;  ichijō): Also, single vehicle, Buddha vehicle, one Buddha vehicle, one vehicle of Buddhahood, or supreme vehicle. The teaching that leads all people to Buddhahood. It is taught in the Flower Garland Sutra and other Mahayana sutras, but the Lotus Sutra places the greatest emphasis on it. The Buddha’s teaching is compared to a vehicle ( yāna) that carries one to a particular state of enlightenment. In accordance with people’s capacities, the pre-Lotus Sutra teachings expound and emphasize the voice-hearer vehicle (shrāvaka-yāna), which leads one to the state of arhat; the cause-awakened one vehicle (pratyekabuddha-yāna), which leads one to the state of pratyekabuddha; and the bodhisattva vehicle (bodhisattva-yāna), which after many kalpas of practice leads one to Buddhahood. The voice-hearer vehicle and the cause-awakened one vehicle are together termed the two vehicles, and with the addition of the bodhisattva vehicle, the three vehicles.

尽管有上述表达，但题目中的“一乘门”，个人认为并不是指具体的“一乘”，而更多的是想表达“佛门”的意思，据此此句可译为“There is the fate that makes one become the Buddhist, only the blessed can get the gists of Buddhism”，供参考。