The church represents a safe haven where Johnny and Ponyboy can be themselves and share their lives with each other. They could say things they couldn't normally say around the gang., because they are secure in the church, where God is there with them.
The Blue Mustang represents the Socs' wealth, power, and authority over the Greasers. Mustangs are expensive, and the Socs use them to intimidate and exercise authority over the poorer Greasers.
Mickey Mouse, the horse that Soda loves, connects to the boys' youth. When the horse was sold, Soda was devastated. He shared a special bond with the horse that he could not duplicate. This represents how the Greasers are not rich enough to have things that they want.