He was an Anglican clergyman, until 1648, or possibly a lay preacher from Berkshire with little formal education. He is known as an associate of Abiezer Coppe, who wrote an introduction to Coppin’s 1649 Divine Teachings. Christopher Hill considers that Coppe took most of his theology from Coppin.
He was constantly in trouble, well documented in pamphlets, arising from the 1650 Blasphemy Act. The authorities treated him leniently in the period 1651 to 1651.
He was imprisoned in December 1655 as a Ranter, a term which is now contested in historiography, after a disputation in Rochester Cathedral. Thomas Kelsey, one of Cromwell's major-generals then based at Dover, took a harder line with Coppin than previously, imposing six months in jail. He defended himself, writing from Maidstone Prison a pamphlet A Blow at the Serpent. Another account was that of Walter Rosewell, pushed out as vicar at Chatham, Kent in 1649, in The serpents subtilty discovered.
Coppin's work provoked Edward Garland, vicar at Hartclip (Hartlip, Kent), to reply in kind in 1657, accusing Coppin of heresies. The pamphlet exchange was extended by Coppin's Michael opposing the dragon (1659).
He is sometimes presented as a ‘moderate’ Ranter, or philosopher of Ranterism. Christopher Hill shaded his opinion to ‘near-Ranter’.