If there is no afterlife, then promoting justice sometimes requires acting unjustly.
Promoting justice never requires acting unjustly.
So, there is an afterlife.
In support of 1, just think of cases where it looks like great injustices can only be stopped by minor injustices. Perhaps the only way to get an unjust dictator out of power is to spread the rumor that he is unfaithful to his wife. Perhaps the only way to bust a criminal organization is to have an informer make false promises. Of course, these cases presume that this life is all there is. If there is an afterlife, perhaps things are so arranged that all wrongs are righted in some way, so on the whole all will have justice. But without an afterlife, these cases are very compelling.
I think the weakness here is the "requires". There is a normative and a non-normative sense of "requires". Justice non-normatively requires A provided that justice cannot be had without A. Justice normatively requires A provided that in light of justice we are morally required to provide A. My line of thought above established claim 1 in the non-normative sense of requiring, whereas claim 2 is most plausible only in the normative sense of requiring.
Maybe. But I still think that 1 also has some plausibility with the normative sense of "requires" and 2 has some plausibility with the non-normative sense, so the argument as a whole has plausibility when "requires" is read consistently. The argument raises the probability of the conclusion. By how much, I do not know.