In chapter 5, Naselli and Crowley discuss “Twelve Principles about How to Disagree with Other Christians on Disputable Matters.” #1 is “Welcome those who disagree with you (Rom. 14:1-2).” Here they re-define the weak conscience:
The weak person’s conscience lacks sufficient confidence (i.e., faith) to do a particular act without self-judgment, even if that act is actually not a sin. To him it would be sin … His conscience lacked the confidence (faith) to do those things without self-condemnation.1
This definition is excellent, as is the remaining discussion, which is based on it. They go on to describe weakness and strength as a spectrum2 extending from permissive (strong) to strict (weak). This is the pattern for the rest of the book, where “weakness” is treated as strictness, not theological immaturity.