This is another excerpt from my paper, THE DEVELOPMENT OF BAPTISM BY BAPTISTS IN SEVENTEENTH CENTURY ENGLAND, for my Texts and Traditions 2 class at Truett Seminary this semester. The first one, what Baptism means for the church today can be found here.
This is more of the history about what Baptism meant.
The Baptist identity that developed out of the English Congregationalist Churches during the Puritan and Separatist movements against the Church of England in the 16th and 17th centuries. For the Baptists of the 17th century, baptism was primarily a matter of ecclesiology. Like the Puritan groups that saw the established church as corrupt and the separatist groups that saw the need to leave altogether, Baptists were concerned with creating a pure church. For them, believer’s baptism was the communal identifier that they belonged to the true church.
In 1609, John Smyth and Thomas Helwys were members of the first church covenant based on believer’s baptism. Smyth believed that, “The Saynts as Kings rule the visible Church.” Smyth’s congregation and the early General Baptists “came to erect a new church by baptism” as the only valid basis for church covenant. This emphasis on the pure church and General Baptist closed membership extended so far that despite the similarities between different Baptists, those transferring membership had to be rebaptized, “Because You were baptized into the wrong Faith, and so into another Gospel.”
The Particular Baptists, formed out of the Jacob-Lathrop-Jessey church during the 1630s also reasoned for a believer’s baptism. While both John Dupper, 1630, and Sam Eaton, 1633, may have practiced believer’s baptism, the first definitive Particular Baptist Church evidence supports is of John Spilsbury’s congregation in 1638. According to H. Leon McBeth, these Particular Baptists had different motives. Being strict separationists already, they denied the validity of any sacraments administered by the Church of England. They all had to be baptized a first, legitimate time.  The multiple exoduses that pushed the Jacob-Lathrop-Jessey church into the Particular Baptist identity included people such as Samuel Eaton, John Murton, and Mark Luker. The Kiffin manuscript of church minutes, named after later Jacob-Lathrop-Jessey pastor William Kiffin, in 1640 described the moment the Jacob-Lathrop-Jessey Church came to the conclusion of believer’s baptism and baptism by immersion at the same time. Baptism, they claimed in the document church minutes, “Ought to be by dipping ye Body into ye Water, resembling Burial & riseing again. 2 Col 2.12. Rom: 6.4.” Though Richard Blunt baptized the pastor, Henry Jessey, the church continued to be of mixed membership at times.
By 1660 the General Baptists had come to the same conclusion in the General Baptist Confession of London. A 1651 General Baptist confession noted the form of baptism understood in scripture “was to go into the water, and be baptized.” The first clear affirmation of General Baptists using immersion was the Standard Confession of 1660. For the 17th century, language concerning baptism as a sacrament or an ordinance was interchangeable for Baptists, and that creeds usually adopted one word out of consistency. It was in fact the General Baptists that used sacramental language to describe baptism and the Lord’s Supper.
 For the sake of space, I’m adopting Winthrop S. Hudson’s views on the origin of the Baptists. I could be entirely wrong, but this is also a practically useful decision because the modern Baptist church is more heavily influenced by Particular Baptists- also see David Bebbington, Baptists Through The Centuries : A History Of A Global People (Waco Tex.: Baylor University Press, 2010), 30–31.
 John Smyth, “The Differences of the Churches of the Seperation (1608),” in The Works of John Smyth (ed. William T Whitley; Cambridge: Cambridge University Press, 1915), 1:273, 274; in Bebbington, Baptists through the centuries, 33.
 John Robinson, William Allen, and John Waddington, The works of John Robinson: pastor of the pilgrim fathers (J. Snow, 1851), sec. 3:168; in Bebbington, Baptists through the centuries, 34.
 Luke Howard, A looking-glass for Baptists being a short narrative of their root and rice [i.e. rise] in Kent wherein the erronious spirit of Richard Hobbs (pastor of the Baptists in Dover, with some others ([London: s.n.], 1672), 5.
 This is not mentioned as a reason in Bebbington’s book, whether out of omission or disagreement I don’t know.
 Leon McBeth, The Baptist Heritage (Nashville: Broadman Press, 1987), 45.
 W.L. Lumpkin, Baptist Confessions of Faith (Judson Press, 1959), 182.
 Curtis Freeman, “‘To Feed Upon by Faith’: Nourishment from the Lord’s Table”, n.d., 200–206; Anthony Cross and Philip E. Thompson, eds., Baptist Sacramentalism (Carlisle Cumbria ;;Waynesboro Ga.: Paternoster Press, 2003); in Brian Brewer, “‘Signs of the Covenant’: The Development of Sacramental Thought in Baptist Circles,” Perspectives in Religious Studies 36, no. 5 (Winter 2010): 410.