I didn't find anything about slandering the Dhamma, but I did come across a reference to Samyutta Nikaya 6:9:
Does this refer to past slander of the Buddha, or is it irrevocable? The person mentioned in the passage did not appear to have changed his views.For more than two quinquadecillion years,
and another five quattuordecillion years,
a slanderer of noble ones goes to hell,
having aimed bad words and thoughts at them.” (Sujato translation)
I'm concerned because I used to be a fundamentalist Christian who thought that all other religions, including Buddhism, were wrong. I cannot honestly remember if I spoke bad words against the Buddha based on these erroneous beliefs, but it would have been in my mind. I do remember saying that the First Precept was impossible, but that was due to not understanding cetana.
Over a year ago I took refuge and converted to Buddhism formally, and have called myself a Buddhist from then on. I am opposed to conservative Christianity and would like to see it decline and be replaced with Buddhism.
Let's say someone had definitely committed slander of the Buddha - imagine they were a Christian missionary who spent extensive amounts of time in countries like Thailand, Cambodia and Myanmar. This person attacked Buddhism and the Buddha frequently while claiming that Jesus is the only way to be saved, and made a siginificant number of converts to their form of Christianity. Imagine they later start reading books critical of Christianity and realise it's not true. They remember what people in these countries had told them about Buddhism, look into it and eventually take refuge. Can such a person progress on the Buddhist path and get a good rebirth in their next life (where they may be able to become a monk and a sotapanna), or has their slander of the Buddha during their time following a wrong view doomed them to zillions of years in Avicii?
A reference is also made to this chapter, which seems to come from the Taisho Tripitaka. I'm guessing it thus has no relevance to Theravada Buddhism, unless there is a parallel text in the Pali Canon.