'Behold Israel after the flesh, do not those who eat the sacrifices have communion with the altar?'
His second example is the oneness with the altar, and all that it meant, of those in physical Israel who ate of the sacrifices offered on that altar. This was important because it paralleled exactly the worship of idols in the offering of a sacrifice and then partaking of it. As they ate of the sacrifices they were one with the altar because that was where the sacrifice had been offered, and they were one with all who participated of the meal, and one in benefiting from the efficacy of the sacrifice. They as it were ate before God (compare Exodus 24:10-11), and were seen as under His sovereignty. His point here is that in the same way if you participate of the sacrificial meat in the temple you are, at least in the eyes of others, uniting yourself with the sacrificial offering which was made to the god from whose altar the meat came. Thus you are making yourself at one with the altar of whichever god is in mind, and therefore professing yourself as under his jurisdiction.
'Israel after the flesh.' That is, physical Israel. We have here another reminder that the church is the true spiritual Israel. To suggest that this refers directly to the worship of the molten calf is to read too much into the wording. Had Paul meant that he would have made it quite clear. Rather he is making a point from true ancient worship.