Most of what is titled "Landmarkism" in Kevin Bauder's Baptist Distinctives book is just biblical and historical belief and practice. It didn't originate with the men who began to be called "Landmarkers" in the mid 19th century. His next section, he titled, "Involvement with Non-Baptists." Bauder reports that Landmark Baptists advocated not having unbaptized individuals to preach to or speak to their assembly, even if they claimed a true gospel and a real profession of faith.
Bauder quotes Dagg, who expresses the point that unbaptized people shouldn't be accepted either into the membership of the church. Dagg says that Baptist churches can treat these professing believers from other denominations like weaker brothers, even though they can't admit them into their churches. Bauder treats the Dagg quote like he agrees. You can consider men from infant-sprinkling denominations to be weaker brethren.
The weaker brother teaching in 1 Corinthians 8 and Romans 14 is about Christians, not that are disobedient to scripture, but with a sensitive conscience to extra-scriptural scruples. No one is permitted just to use "weaker brother" however he wants. The weaker brother is a particular type of young believer. Infant sprinkling isn't a weaker brother issue. This is how scripture is manipulated in a pragmatic fashion. It undermines biblical teaching on baptism, which dishonors God.
Bauder implies that he agrees that unbaptized people shouldn't be accepted into membership, but in addition to that negative responsibility, he says that there is a positive one to members of other denominations, like the ones that sprinkle infants. Bauder says that Baptist churches have a responsibility to recognize them as members of a universal church. Bauder says that Baptist churches have a duty to receive professing Christians who are not really baptized, because they are members of the universal church. Bauder writes:
Brothers can welcome, encourage, support, sympathize with, and pray for one another even when they do not agree on all matters of church order.He doesn't offer any scripture to back up this point. There isn't any. I see it as sheer sentimentalism. Believers have a responsibility to separate from false doctrine and practice. The twisting of the weaker brother teaching does not justify it. Believers have a greater responsibility to separate from those who call themselves brothers (1 Cor 5, 2 Thess 3).
With his next three paragraphs, Bauder lectures Baptists on how to treat men from different denominations, who differ in doctrine and practice. He says, they shouldn't think they're stubborn or stupid and they shouldn't misrepresent their positions. Separation, however, isn't unloving behavior. You should first try to help someone understand the truth. Talking about issues with a Presbyterian, for instance, isn't fellowship. It's ministry, doing spiritual warfare. If he is open to listen, you can spend time together, at the same time ensuring him that the differences aren't minor. They matter.
When the Apostle Paul wrote to speak the truth in love, he also meant that it is loving to speak the truth. Not speaking the truth is not loving. Another doctrine in the Bible is the perspecuity of scripture. You really can know what it means and so you should act like it. With that biblical presupposition, you can conclude that men are being stubborn, that is a problem of their will and not their intellect. It's not because they can't understand, and we can see in scripture that stubbornness is a prominent reason. It could also mean that they really aren't saved, which is why they stay in perpetual disobedience to the plain truth.
Bauder encourages cooperation of men in various denominations in "interdenominational service organizations." Again, he provides no biblical basis for that counsel. It seems to rest entirely on some extrapolation from or expansion of the idea of a universal church or catholicism. It is a recipe for the toleration of and then spread of false doctrine and practice.
The assumption of Bauder is that churches need parachurch organizations, that churches alone are not sufficient for God's work. The sufficient Word of God reveals a sufficient institution for all of God's work, the church alone. Taking the truth outside of the church -- with its pastor, ordinances, and church discipline -- causes the corruption of doctrine and practice. Another assumption in light of the chapter is that missing out on these interdenominational service organizations is some type of tragedy of Landmarkism.
To end the chapter, Bauder advises the practice of inviting Presbyterians and Methodists to preach at your Baptist church. He suggests that successful accomplishment of the interdenominational meeting requires a "gentleman's agreement" where the different doctrines are not preached. The various factions keep only to doctrines and practices they can agree on. Bauder abruptly ends a chapter on Landmarkism with a paragraph admitting that there is nothing laid out in scripture to guide such a coalition of varied beliefs and practices, so that each Baptist church must figure out on its own what is a comfortable level of involvement with people who don't believe like it does.
In fact, the Bible has a lot to say about how to deal with false doctrine and practice, and cooperation is not part of it. With the way Bauder finishes, his chapter against Landmarkism reads like a support of interdenominational fellowship. It reads like a defense of fundamentalism. Bauder seems like more of a fundamentalist than he does a Baptist. I don't know of a biblical defense for fundamentalism. However, the Bible does teach us the terms and conditions for fellowship with other believers. Even if you defend interdenominationalism with universal church teaching, you will disobey the Bible in several different ways, because universal church doctrine itself clashes with the Bible.