I think what you're attempting is a trompe l'oeil style of painting. There is a lot of information on the web if you just enter the above words and some really magnificent work to be seen by modern artists.
As for your painting, you might need more variation in the colors to make them receded and also to come forward. The shadow of the cross would be darker in areas and not all of the same shade.
It's an interesting style and takes a different way of 'seeing'.
When life hands you lemons, ask for tequila and salt and call me!
FAQ Oil Painting