When a person commits a mortal sin out of ignorance, when does he commit a mortal sin, and in what does it consist? Take the case of a married person believes that contraception is not intrinsically wrong, and consequently judges in a particular case that he or she is obliged to use artificial contraception in order to ensure that the married couple can educate well the children they already have. According to Aquinas's account, the choice and act of contraception in this case is either no sin at all, or it is a mortal sin (from the analogy of contraception, he seems likely he would consider this generally to be a mortal sin). Let's suppose that the ignorance is not invincible ignorance, and the choice and use of contraception is a mortal sin.
There are two ways we might interpret the case: (1) the mortal sin might be found in the grave and voluntary neglect to acquire the knowledge that would have kept him from sin, or (2) the mortal sin might be in the act that proceeds from ignorance, even if the ignorance itself was not a mortal sin; in this case one would have to say that the actual disorder arises due to the ignorance, yet the voluntariness of the disordered act (which is necessary in order for it to be sinful) is on account of the prior neglect to acquire knowledge. A similar pair of accounts could also be made of sins committed out of passion, though it is not as evident as in the case of ignorance: (1) the sin could be attributed to the neglect to resist the passion, or (2) to the act that follows upon the passion, inasmuch as this disordered act is voluntary by reason of the lack of resisting the passion, (even if the failure to resist the passion in itself was not a mortal sin).
Let us first assume the second interpretation of the case at hand. Now, in the period of time when the choice for contraception is made, it is not in fact possible to rectify the ignorance. Therefore the voluntariness of the ignorance and the disorder of the choice consequent upon the ignorance must be related to a prior neglect over a longer period of time. In this case and according to this interpretation we indeed have an individual mortal sin chosen and committed at a particular time, yet at least one of the elements necessary in order for that sin to be mortal, namely the voluntary neglect to form his or her conscience, is not to be found simply in the short period of time leading directly up to that choice, but over a relatively long period of time.
What if we take the first interpretation? Then we understand the very neglect of forming the conscience to be the mortal sin, and would probably understood there to be formally no additional wickedness in the act that follows from the ignorance. (That is, when someone acts according to a malformed conscience, and does an objectively disordered act that he believes to good and perhaps morally obligatory, this act does not have any new wickedness. If it is called an additional sin, it is called so because it is materially a new act that shares in the prior wickedness of the neglect to form the conscience, of which it is the expression and manifestation.) According to this interpretation, when is the person guilty of grave sin for neglecting to form his conscience? We would have great difficulty in pinpointing an exact time. More importantly, even if we were able to pinpoint an exact time when the person became guilty of grave sin of neglect to learn what is truly good and evil, there would in many cases be nothing particularly special about that time. (Similarly, if someone steals $20 from a coworker every day, there would come a point where he is guilty of grave wrong against his coworker, and a mortal sin, without there necessarily being any special new decision "to be a thief" or "to be unconcerned for others' property.") A grave sin of neglect to form one's conscience is committed at a particular moment only insofar as that moment morally carries along with it a long previous chain of neglect of which it is, so to speak, a finishing link. According to this interpretation, too, then, the mortal sin of neglect of conscience, is sinful only on accord of a long period of neglectful behavior.
According to either interpretation it seems necessary to admit that there are mortal sins whose sinfulness cannot be analyzed in terms of a brief deliberation and choice, or even a series of choices over a few hours, but whose sinfulness and voluntariness has to be related to a relatively long period of time, measured possibly in months or years. I am not aware of Aquinas explicitly recognizing such a dependence of sin upon previous voluntary acts, but he does speak about something similar in regard to conversion to God. Having spoken of how God can move someone immediately to a complete conversion of heart and to charity, Aquinas goes on to say "Sometimes, however, one act disposes someone to the infusion of grace only by a remote disposition, and the following act disposes him still more, and so on, so that the last disposition is attained out of many good acts, insofar as a subsequent act always acts in virtue of all the preceding ones" He compares the completeness of conversion to the gradual disposition of a chip of stone to be broken off from it: "as is evident in drops of water hollowing out a stone, where it is not each and every drop that takes away something from the stone, but rather, all the preceding ones are disposing the stone to be hollowed out, and one last agent, in virtue of all the preceding ones (insofar, that is, as it finds a matter disposed through the preceding drops), completes the hollowing out.