Is it ever okay to say "No" to a ministry that is doing the work of God? I think the answer is "Yes." Is God wanting us to say "Yes" to every ministry opportunity that crosses our path? Here I think the answer is "No."
I have experienced the pressure from well meaning individuals that want me to become involved in their ministry. It may be sending money to the starving children, or showing up to an event and working to make the event happen. The work may be God's design and it may be a very good work, but it is not always my place or my call from God.
My offer to serve is generated by the voice of God calling me into that service, not the voice of a friend. Saying "No" is difficult. I imagine that the person asking for help will somehow think that I am rejecting them, passing judgement on their ministry, or simply not hearing God though my answer is none of these. Unless we all begin to understand the working of God in each heart to do different things we will continue to struggle with saying "No."
God does not expect me to say "Yes" to another call. He only expects a "Yes, Lord." When He calls my name and asks me to serve. It may be one area of service that He calls me or it may be many. He knows me and He knows the abilities with which He has equipped me. He will not over burden me or my family with too much to do. He will not fill up my calendar so I can look like a devoted follower of His. He desires that I am a devoted follower not a Pharisee that looks right on the outside but dead on the inside.
So, if I say "No" to you, please understand that I must follow Him. Though I may want your works to be a success I am not the person you need to fill your vacancy.
Father, Thank You for knowing my limitations and my strengths. I need help to only do those things that You have called me to do. Open my eyes to see the goals set before me by Your plan for my life. Remind me often of my commitment to serving You alone and not man. Thank You for being my Guide and my Protector. Let my yes be yes and my no, no.