I wonder whether Lendl told Murray that much like some players believe God is on their side, Murray should try playing for something else but himself. Perhaps Murray chose to play for the children who died in the Dunblane massacre. One person on this forum, I forget who, brought up the possibility of survivors guilt, which is why I thought this as a possible theory.
I'm also starting to reconsider whether it's possible he's turned to God but I have a vague memory of him making a comment in an interview once that quite strongly suggested he was not the religious type.
Either way, I'm inclined to believe the gesture was spawned from something Ivan has told him.
Well if you believe the report in the Express, when Andy sat down at the end he threw his head back, looked towards the sky, his lips were seen moving slightly and he apparently mouthed "Amen" at the end before standing up.
I didn't notice what he did but I'm pretty sure his reaction was due to exhaustion rather than any religious belief, although one can never be sure. Also the Express does tend to be a little purple at times and there were no other comments to substantiate its report.
Like you I'm prepared to believe this gesture is a Lendl-generated thing, whether religious or otherwise, and one which we are never going to be told about.