﴿ وَكَانُوا۟ يُصِرُّونَ عَلَى ٱلْحِنثِ ٱلْعَظِيمِ ﴾

“and would persist in heinous sinning,”

(And used to persist) in the life of the world (in the awful sin) i.e. associating partners with Allah; and it is also said that this refers to the lying oath.