Turretin is clear that discussion of this question is motivated, or “moved,” by engagement with the Roman church. And to tackle the question he makes a series of distinctions in true scholastic fashion. He begins by addressing the issue of catechumens, and it is here that he makes his primary distinction:
“The church can be regarded in two ways: either as to external state and visible form, or as to internal and invisible form.” (18.4.3)
This is your classic Augustinian distinction between visible and invisible church, and it has been used to solve many an ecclesiological problem throughout theological history. For Turretin, it makes it possible to speak of catechumens as members of the invisible church even if they have not yet undergone the sacrament of baptism to become part of the visible church. He offers three lines of argument in support of this conclusion. First, Turretin appeals to the “converted thief,” by which I take him to mean the thief who was said to be crucified with Jesus and responded positively to Jesus. Here is someone who was not baptized but who seems to have been included within the invisible church and therefore saved. Turretin also goes after Bellarmine here because Bellarmine says that catechumens are part of the church potentially but not actually. For Turretin, either you’re in or you’re out, and Bellarmine’s solution is no solution at all. Second, Turretin collects a number of biblical examples of folks who “were reckoned in the body of the spiritual people [of God / the covenant] and possessed the same blessings by faith” even though they had not received “the symbols of the covenant” (18.4.5): Melchizedek, Job, the eunuch in Acts 8, and “innumerable martyrs who died before they had received baptism.” As a note, the ancient church used to speak of such martyrs as having undergone a baptism of blood—Turretin doesn’t mention this terminology, so you’re welcome. Third, “baptism is not necessary absolutely to salvation” (18.4.6) since the early church recognized that desire for baptism or martyrdom suffices if the normal rite is unable to be performed. This clearly suggests to Turretin that inclusion in the external / visible church by way of its standard rites cannot be equated with membership in the internal / invisible church. Here are a couple nice quotes about baptism and the church:
“It is one thing to be gathered into the visible church, which is done by baptism; another into the invisible, which is done by faith” (18.4.8); “Baptism makes a Christian by profession, but not by internal truth” (18.4.10).
After addressing catechumen, Turretin turns to address those who have been excommunicated, and he makes another set of distinctions. Excommunication can be of three classes: “unjust,” “just and lawful,” and “absolute and total” (18.4.11). The first doesn’t matter because it is done in error; consequently, those excommunicate in this way may be put out of the external / visible church but they remain part of the internal / invisible church. The third doesn’t matter either because it is impossible given that excommunication only pertains to the external / visible church. Turretin comes up with a nice quote from Augustine as evidence for this view (18.4.14), and even brings in a quote that seems to demonstrate Bellarmine’s agreement (18.4.17).
Finally, Turretin addresses schismatics. He does so rather briefly, and I will quote it in full. Presumably he will discuss this issue at greater length elsewhere:
“To touch briefly upon schismatics, we readily grant that they who rashly and unjustly secede from the true church, by lacerating its unity, do not belong to the church. But we reject those who spitefully traduce us with this false name (because “[we] are unwilling to be in subjection to the Roman pontiff,” as Bellarmine has it) as being excluded from it.” (18.4.18)