click to enlarge (gets very big)
We'll leave the comments to Dali himself, along with a chunk out of the wiki page on the painting.
The painting is known as the "Christ of Saint John of the Cross," because its design is based on a drawing by the 16th century Spanish friar Saint John of the Cross. The composition of Christ is also based on a triangle and circle (the triangle is formed by Christ's arms; the circle is formed by Christ's head). The triangle, since it has three sides, can be seen as a reference to the Trinity, and the circle may be an allusion to Platonic thought. On the bottom of his studies for the painting, Dalí explained its inspiration: "In the first place, in 1950, I had a 'cosmic dream' in which I saw this image in colour and which in my dream represented the 'nucleus of the atom.' This nucleus later took on a metaphysical sense; I considered it 'the very unity of the universe,' the Christ!"
For what it's worth, your humble scribe has visited the museum where this painting hangs on many occasions. Mainly to see this work, really. I don't know where they hang it these days, but back then it used to be at the end of a very long corridor on the first floor and the best way of reaching it was to walk the corridor from one end to the other, watching the painting get bigger and bigger in front of your eyes. It's awe-inspiring.