Car will not start

My car will not start. I have replaced the starter, battery, and alternator and still nothing. I have plenty of power, but it will not start. Before it stopped working I would start it up and it would sputter, got a message and had it checked said I needed to replace the MASS air flow censor. I did that and it stopped for a about a week and started up again. The engine light came on again and I had it checked said the same thing, but it kept starting. Then one day it would not start. I just replaced the alternator but it is still not starting. I was told it could be the engine relay, but I do not want to buy it and have that not work and I would have wasted more money.