Which was most significant about Maryland's act of toleration

I believe this may be the answer your looking for: The Maryland's Act of Toleration inspired the growth of religious freedom in the colonies, as it allowed tolerance to non Puritan Christians in the colony.

The law became a model for other North American colonies, as later came other laws that ensured tolerance and religious freedom, such as the Holy Experiment in Pennsylvania, and Religion Laws in other colonies like South Carolina, which have been influenced by his example.

This act also influenced the freedom of religion that was later legislated with the creation of the United States of America.