Illustrate a piece of art that evokes a sense of devotional spirituality based on the concept of Matthew 18:20, which represents unity and prayer. Picture this concept in the aesthetic of the Art Nouveau era, characterized by intricate linear designs, natural shapes, and ornate decorative elements. The mood of the picture should be reverential, yet serene. The formulation should refrain from using any text or words, instead conveying the message through symbolic and allegorical imagery. Note the artwork should be respectful and considerate to Christianity.