Approach a religious-themed bulletin board project the way you would any other themed bulletin board; begin with the message or purpose of the board, and then build the rest of the design around it. Consider the audience who is going to see the board: children, adults or all ages. Then, think of eye-catching artwork, phrases or color schemes that suit the board's purpose.
If designing a board to reflect a particular Bible story, such as Noah's ark, begin with a centerpiece. In this example, you might choose a large ark made of construction paper or lightweight card stock. Then, think about the title of the board. Keep it short, so the lettering is eye-catching and large enough to be seen from across a room. Titles could be "Noah's Ark," "Two by Two" "God's Promise" or something else highlighting the message from the story you want to convey. Then, add animals, a dove and a rainbow. Adding these elements ensures that each of the main themes of the Noah's ark story are "readable" at a glance.
If the bulletin board is for adults or for all ages, use more a more mature, restrained design. Possible motifs could include a cross, star of David or other symbol meaningful to those who look at the board. If the board is one that changes throughout the year, consider using a color scheme in keeping with the liturgical calendar, such as purple during Lent, or green during Ordinary Time.
Just because a board has a religious theme, it does not mean its design cannot be subtle. Careful use of colors, artwork and titles can inform your audience while engaging them at an appropriate level.