William Booth (1829 – 1912) was the founder of the Salvation Army, an organization that was first created in England to bring Evangelical Christianity to the people living in poverty in the East End of London. Lindsay's poem is written in gratitude to Booth and the Salvation Army, which helped him when he was destitute.
The people Booth leads into heaven are those who are poor, outcast, and marginalized, precisely those that Jesus is portrayed as ministering to in the New Testament. These included lepers, alcoholics, drug addicts, prostitutes, convicts, the insane, and the destitute.
Lindsay portrays a Heaven which is very much an idealized version of Booth's Salvation Army, in which the motley crew following Booth are made clean and whole and healthy, and transformed into something looking like a real army on parade in a vast courtyard, as Jesus appears in front of Booth.