The Joads didn't lack biblical leadership; in fact, they exemplified it. What they didn't do was rely on God to solve their problems. When Ma Joad chases away the religious zealot who was talking to Rose of Sharon and chastising her for "hug dancing", Steinbeck is showing the reader that words can do more harm than good if used inappropriately. He shows the reader that actions are more important than words. He shows this through the character of Jim Casy as he realizes that he needs to help unify people, through the character of Tom as he takes on Casy's work after Casy is killed, through the character of Ma as she is continually the pragmatist doing whatever it takes to help her family survive, and finally, through the character of Rose of Sharon as she silently provides nourishment to the starving man in the barn. The religious belief that Steinbeck displays throughout the book is that people need to work together for the collective good.
This is an interesting question. I certainly don't think the Joads turned their backs on God; this is witnessed through grandma's insistance on Casy remaining a preacher. There are examples from the past that the Joad children were baptized, they know Casy from his preaching days, etc. However, I certainly think the Joads have made the assumption that "doing" will get them farther than praying. Someone devoutly religious may have stayed on the farm and waited for God to provide or for a some definite sign of what to do. The Joads, like so many other Okies, took to the roads to take their chances. But again, I'm not sure this constitues a lack of religion.
Yes, they certainly relied on fellow travelers more than the power of prayer, but wouldn't the devout make the assumption that the willingness to help each other was a divine gift? You can probably pick whichever side of the fence to sit on you wish - the Joads weren't attending church services or kneeling down at the bedside to say their prayers every night, but they weren't in the middle of the road cursing God for their plight either.