Explanation:

Ramanuja never criticized Shankara. He criticized only the disciples of Shankara (Yaduchyate Shaankaraih…), who misunderstood the monism and were not spiritually developing. Every Tom, Dick and Harry started thinking that he was already God. Therefore, the truth was revealed to them through Ramanuja. Since, the concept of theism became firm, nobody became atheist. Those who realized the truth climbed the step of devotion accepting dualism. Those who were not convinced remained in monism only and did not become atheists. Subsequently, Shankara sent His another servant Vayu, an angel, in the form of Madhva to preach the third and final step of practical devotion. Madhva, Himself claimed as the incarnation of Vayu and Vayu is only one of the angels. Dualism is not a new invention of Ramanuja and Madhva. It was already stressed in the prayers by Shankara. Ramanuja and Madhva stressed the devotion and dualism, which are parts of the total spiritual concept of Shankara only.