I hope this message finds you well, present circumstances excluded. I am a licensed attorney with over a decade of experience handlings matters similar in nature to your situation. It is a pleasure to assist you today.
The answer to your question depends on whether or not you have a lease or contract with the church to have the church grounds for the stated period of time. It also depends on the language in the agreement, if there is one, as to what rights to exclusivity are enjoyed by you while on the premises.
If there is no lease agreement or contract, then as a legal matter, the pastor and church have the right to pray before the concert if they so choose and there is really nothing you can do to prevent it, since you are considered to be invitees on their premises.
If there is an agreement for the space and time in writing, then the premises, dependent on the nature of the language within the agreement, should allow you to kindly ask them to refrain from this activity as the premises are yours to utilize as you wish during the agreed upon period of time.
Interestingly, the church could decline to renew any agreement that is in place going forward if they are not allowed to pray as a component of the deal moving forward.
In short, the enter issue is subject to whether or not there is a lease/contract in place, primarily, and to a lesser extent, what that agreement says or allows the church do to when you have the premises to yourself. If there is no lease or contract, then the pastor can continue to do what he wants.
Let me know if you have any other questions.
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Best wishes going forward!