Can a saved person fall out with God? Sure. I did at one time in my life. People who do though usually do from some crisis that has occurred and are eventually brought back into the fold. Here IMO actions are very important cues. Are they happy or appear lost and searching? If someone was truly saved they will not have peace until they are right with the Father and the Son.
Rejection of God is quite another. Within OSAS you either DO believe that Jesus is who He IS, or you were NEVER "saved".
Will a saved person have doubts? Almost certainly. Sin? Yep. Become "less religious" almost to the point of agnosticism? It could happen. Every REALLY believe "Jesus Christ is NOT God"? I think it's possible, you shouldn't.