State leaders who supported state religions might have favored the establishment clause to:
a. ensure that the national government did not interfere with the state governments’ right to establish a religion.
b. separate the religious establishments and the government bodies.
c. ensure that the national government could firmly direct and influence the public expression of religious ideas.

a. ensure that the national government did not interfere with the state governments’ right to establish a religion.

Explanation:

Prior to the creation of the establishment clause , Churches have a really huge influence in United States government. Establishment clause basically prevents the government from making any law that force others to conform to a certain religion,.

But, the words that they used in the clause is a little bit confusing.

This made State leaders who supported state religions misinterpreted what it actually means.  They thought that the establishment clause meant that the national government wouldn't be able to intervene when they want to established state religion. It's an irony because establishment clause was intended to prevent the government from establishing state religion.

Sorry bro, this is the English subject