Does Iman increase and decrease

From here we always and forever say as we were taught by our Salaf: “It is mandatory upon you to stick to how it was at the old time (i.e., the time of the Prophet and the Sahabah).” At the time of the Sahabah, there was nobody that said: Iman does not increase or decrease. That's because this contradicts the Quran and the Sunnah. However, about a hundred years after them, there was someone who had an opinion and erred (in it), and he was a knowledgeable and virtuous man. He saw that it is impossible for Iman to increase and decrease, but only its effect increases, which are righteous actions. And Allah always and forever says that Iman increases thus good deeds increase. Then, this opinion that was given by some of the people of knowledge was taken as a religion, and this is one of the calamities of Muslims! How is it taken as a religion whilst the religion is al-Islam, and al-Islam is al-Quran, and the Quran explicitly says that Iman increases and decreases! “فزادهم ايمانا” "But It (only) increased them in Iman"