Because the only way efficiently to bring our actions into perfect harmony with objective morality is to follow the safe opinion so long as the less safe opinion has not acquired moral certainty. This is the only way of preventing all serious danger of committing material sin and consequently is the only way of observing perfect harmony with objective morality. However, since Rigorism is universally condemned, the argument must be rejected, and the principles of Probabilism must be adopted which hold that it is sufficient to observe harmony with objective morality insofar as this is known with moral certainty (cf.
The passage argues that following safe opinions is necessary to achieve perfect moral harmony, as long as less safe opinions haven't gained moral certainty. This approach prevents serious sin and ensures alignment with objective morality. However, the argument is rejected due to Rigorism's condemnation, leading to the adoption of Probabilism which emphasizes observing harmony with objective morality only when certain.