Spiritual addition is the principle that someone can bring another person to the point of conversion - that is, they choose to follow Jesus and become a disciple of his. This is an intentional act of the will. We should rejoice anytime someone is brought to a living faith, but we should never stop there. In many ways, this is the bar of success in most Catholic circles. We seek converts and disciples (not a bad thing!). Still, this isn't the bar Jesus set for us as his followers.
We see at the very beginning of Jesus ministry that he called the fishermen to be fishers of other men. He said, "Follow me, and I will make you fishers of men." (Matt 4:19). The first step is to "follow". This decision to follow makes one a disciple. The next step is to be formed into a "fisher of men". This makes us a disciple-maker. Yet, it doesn't stop there. We are to go onto not just make disciples, but disciple-makers. This is spiritual multiplication.
If you were to become a great evangelist, you might have the opportunity to bring hundreds (or thousands like the great Saints of old - e.g., St. Paul, St. Francis Xavier). You could personally evangelize the crowds of those around you and heaven would rejoice. Yet, there is a better way. To deeply invest in a handful of people at one time. Thus, forming them to be disciple-makers as well. This is the model of Jesus! He didn't spend the bulk of his time ministering to the crowds, though he did do so sometimes. Rather, he took 12 followers and made them into "fishers of men" who made other "fishers of men".
The math bears this out. If you become one of the greatest evangelists of all time and are able to lead 1,000 people to faith every year for 36 years, you would have 36,000 followers of Jesus. Amazing, isn't it?
Yet there is something much better, if you led 3 people to Jesus, personally discipled them, and raised each person to evangelize and disciple 3 others, and then all of those disciple-makers reached 3 others, then at the end of 36 years, we will have 1,048,576 followers of Jesus!!!
That is what spiritual multiplication looks like and what we are called to.
If our goal is to "make disciples of all nations" as we are told to by Jesus, then we need to aim for spiritual multiplication, not just spiritual addition. St. Paul modeled spiritual multiplication, by investing in Timothy, Titus, etc. He spells it out in one of his letters to Timothy:
"What you have heard from me before many witnesses entrust to faithful men who will be able to teach others also." -2 Tim 2:2
If you don't know where to start, then I encourage you to reach out to us for help. This is why Catholic Missionary Disicples exists!