Conversation so far:
What was A. C. Bhaktivedanta Swami Prabhupada's religion?
A. C. Bhaktivedanta Swami Prabhupada was Hindu.
What was A. C. Bhaktivedanta Swami Prabhupada's training like?
In 1922, when A. C. Bhaktivedanta Swami Prabhupada first met his spiritual master, Bhaktisiddhanta Sarasvati Thakura, he was requested to spread the message of Chaitanya Mahaprabhu in the English language.
Did A. C. Bhaktivedanta Swami Prabhupada spread the message of Chaitanya Mahaprabhu in the English language?
In 1944, A. C. Bhaktivedanta Swami Prabhupada started the publication called Back to Godhead.
How long did the publicaiton Back to Godhead last?
Did A. C. Bhaktivedanta Swami Prabhupada travel?
From 1950 onwards, A. C. Bhaktivedanta Swami Prabhupada lived at the medieval Radha-Damodar mandir in the holy town of Vrindavan.
Did A. C. Bhaktivedanta Swami Prabhupada teach anyone else?
Through his mission, he followed and communicated the teachings of Chaitanya Mahaprabhu and introduced bhakti yoga to an international audience.
In Vrindavan, what did A. C. Bhaktivedanta Swami Prabhupada do?
A. C. Bhaktivedanta Swami Prabhupada lived at the medieval Radha-Damodar mandir in the holy town of Vrindavan.

Question: why was he called Swamiji?
Self-contained rewrite: Why was A. C. Bhaktivedanta Swami Prabhupada called Swamiji?