In any reflective essay, you are going to be comparing your previous knowledge (gained through reading, personal experiences or general knowledge) of the topic (usually a person, place or experience) with the current experience. Often the easiest way to organize this is to use the introduction to tell about your expectations based on your previous knowledge. Your thesis statement will be the main impression you have of this church or place of worship after attending. That thesis will usually either confirm your previous experience or contrast with that experience. Here are some samples:
Contrast: Although I generally had thought of a church as a boring place to go, this church service was interesting, lively and left me with a feeling of peace and contentment.
Confirmation: My previous experiences in church had made me think of worship as something that calmed my soul and this experience confirmed that feeling.
The body of your paper will explain what happened when you attended and use rich sensory images to describe what you see, hear, feel, smell and think about this moment. The second part of the body section will explain the meaning of this experience in your life. A good conclusion would be to tell whether this experience makes you want to go back.