So, in 1866, Ramakrishna undertook a "sadhana" of Islam and later a "sadhana" of Christianity in order to prove in his own heartspace that all faiths and spiritual perspectives lead to Her. Here Nikhilananda describes his initiation into Islam and Christianity: . . .
In his own being, then Ramakrishna realized the essential unity of all religions at the highest and deepest level. That realization is what gives his teaching on the nature of the Divine Mother such range and authority.— Andrew Harvey, The Return of the Mother