Al-Qurtubi comments on the verse:

“And they swear by Allah their strongest oaths that if a sign came to them, they would surely believe in it.”-6:109.

“And that is because they (the mushrikoon) used to believe that Allah was the greatest deity,and that these deities are only worshipped because they falsely believed that they brought them closer to Allah in status. Just as Allah said about them (that they say): ‘we do not worship them except that they bring us closer to Allah…’”

-Tafsir al-Qurtubi.