Stoning is the most severe punishment in Torah and you can tell what things are more stringent than others if this is said in the Torah to be the punishment. Idolatry is one of these. 

[I was reminded about this because i walked into a breslov na nach sort of place and opened up the mishna in sanhedrin]

but it also occurred to me that things can be idolatry even they do not qualify for the full punishment. this is like on Shabat when there are plenty of things which desecrate the Sabbath even if they do not get up to the level of the full punishment. a simple example i if one picks up an object in a public domain but puts it down in a מקום פטור [a non obligated place].   

So what this means is that there is plenty of stuff which is going on in the religious world which i idolatry even if it does not reach the level of full liability.