Is it sufficient, regarding the rights of one's parents, to introduce them to Islam?

The rights of parents are great, and Allah has coupled His rights with theirs in many verses, and has enjoined good treatment of them and companionship with them in kindness, even if they are disbelievers. Part of their great right over their child is that he should invite them to Islam gently and kindly, encouraging them to be saved from the Fire. Their embracing Islam due to their child's invitation is considered a great reward for them, and we hope that its reward will be greater than the reward of freeing slaves, as stated in the noble Hadith: "No child can repay his father unless he finds him enslaved and buys him and frees him." So what if his son saves his parents from the bondage of polytheism and the punishment that results from it in the Hereafter?