Landmarkism is a "logical system" of ecclesiology. It is a theology concerning the doctrine of the church in the same way Calvinism and Arminianism are logical systems of theology concerning salvation. I will give below what I believe to be the main points or basic substance of this ecclesiological system.
About six or seven years ago, I conducted a study to identify/count independent unaffiliated Baptist churches holding Landmark ecclesiology. To do so I had to develop criteria to identify such churches -- what might be the essence of being "Landmark". The three pillars of Landmarkism (A. C. Dayton, J. R. Graves and J. M. Pendleton) were not agreed in all points. For example, Graves held that New Testament use of the word "church/ekklesia" always meant a local congregation. Pendleton thought that in some cases it meant "the church in the aggregate." I determined the best way to identify a "Landmarker" or church holding Landmark ecclesiology would be to identify the common characteristics of "self-identified" Landmark Baptists from their day to the present. From that I developed what I believed would be the minimum requirements (in combination) it would take to be a "Landmark Baptist church" or "Landmark Baptist". All might not agree with my method or my conclusion, but I offer the results here for comment.
1. The church is a local autonomous body authorized by Jesus Christ to evangelize, baptise, and teach His disciples.
2. Jesus organized His church during His personal ministry, promised its continued existence, and that church (generically) still exists today.
3. Baptism is the immersion of a believer in water by the authority of a local New Testament church; believers who have been immersed by other denominations must submit to baptism by an authorized administrator.
4. The Lord's supper is restricted to baptised believers who are walking in orderly church capacity.
This ecclesiology is most often associated with Missionary Baptists, since the name and the systemization of it were associated with Dayton, Graves, Pendleton and others in the Southern Baptist Convention in the mid 1800s. Some who hold the above ecclesiology prefer not to be called Landmarkers. Among some Missionary Baptists it has never attained common usage, though they gladly own all four points listed above. It is debatable whether it is proper to use this term in reference in to others. Nevertheless, other Baptists hold a similar ecclesiology.