According to section 213 of the Indian Succession Act 1925 the probate or letter of administration for a will is not mandatory for the Christians. This law is also applicable in Mumbai. Hence, you are not bound to take probate under the Indian Succession Act.
Instead of probate you have to file a civil suit. However, if you file a probate the court will not refuse on the basis that a Christian does not require probate under Section 213(2) read with Section 54 (c) of the Indian Succession Act. But the proper procedure is to file a civil suit instead of a letter of administration or probate.