Allah commands His servants to be dutiful to parents, after urging them to adhere to belief in His Tawhid, because a person's parents are the cause of his existence. So he must treat them with the utmost kindness and respect, his father for spending on him and his mother because of her compassion for him. Allah says:

وَقَضَى رَبُّكَ أَلاَّ تَعْبُدُواْ إِلاَّ إِيَّـهُ وَبِالْوَلِدَيْنِ إِحْسَـناً إِمَّا يَبْلُغَنَّ عِندَكَ الْكِبَرَ أَحَدُهُمَا أَوْ كِلاَهُمَا فَلاَ تَقُل لَّهُمَآ أُفٍّ وَلاَ تَنْهَرْهُمَا وَقُل لَّهُمَا قَوْلاً كَرِيمًا - وَاخْفِضْ لَهُمَا جَنَاحَ الذُّلِّ مِنَ الرَّحْمَةِ وَقُل رَّبِّ ارْحَمْهُمَا كَمَا رَبَّيَانِى صَغِيرًا

(And your Lord has decreed that you worship none but Him. And that you be dutiful to your parents. If one of them or both of them attain old age in your life, say not to them a word of disrespect, nor shout at them, but address them in terms of honor. And lower unto them the wing of submission and humility through mercy, and say: "My Lord! Bestow on them Your mercy as they did bring me up when I was young.") (17:23-24) Although Allah orders us to show kindness, mercy and respect towards them in return for their previous kindness, He says:

وَإِن جَـهَدَاكَ لِتُشْرِكَ بِى مَا لَيْسَ لَكَ بِهِ عِلْمٌ فَلاَ تُطِعْهُمَآ

(but if they strive to make associate with Me, which you have no knowledge of, then obey them not.) meaning, if they are idolators, and they try to make you follow them in their religion, then beware of them, and do not obey them in that, for you will be brought back to Me on the Day of Resurrection, and Allah will reward you for your kindness towards them and your patience in adhering to your religion. It is Allah Who will gather you with the group of the righteous, not with the group of your parents, even though you were the closest of people to them in the world. For a person will be gathered on the Day of Resurrection with those whom he loves, meaning, religious love. Allah says:

وَالَّذِينَ ءَامَنُواْ وَعَمِلُواْ الصَّـلِحَـتِ لَنُدْخِلَنَّهُمْ فِى الصَّـلِحِينَ

(And for those who believe and do righteous good deeds, surely, We shall make them enter with the righteous.) In his Tafsir of this Ayah, At-Tirmidhi recorded that Sa`d said: "Four Ayat were revealed concerning me -- and he told his story. He said: "Umm Sa`d said: `Did Allah not command you to honor your parents By Allah, I will not eat or drink anything until I die or you renounce Islam.' When they wanted to feed her, they would force her mouth open. Then this Ayah was revealed:

وَوَصَّيْنَا الإِنْسَـنَ بِوَالِدَيْهِ حُسْناً وَإِن جَـهَدَاكَ لِتُشْرِكَ بِى مَا لَيْسَ لَكَ بِهِ عِلْمٌ فَلاَ تُطِعْهُمَآ

(And We have enjoined on man to be dutiful to his parents; but if they strive to make you associate with Me, of which you have no knowledge, then obey them not.)" This Hadith was also recorded by Imam Ahmad, Muslim, Abu Dawud and An-Nasa'i. At-Tirmidhi said, "Hasan Sahih.