'Stumbling block' in the Bible

Now, brothers, if I am still preaching circumcision, why am I still being persecuted? In that case the stumbling block of the cross has been abolished.

But I have a few [things] against you: that you have there those who hold fast to the teaching of Balaam, who taught Balak to put a stumbling block before the sons of Israel, to eat food sacrificed to idols and to commit sexual immorality.