Irrespective of differences in understanding Deen (way of life) with your parents, there is clear decrees in Quran to honor Parents (17:23-24), 2:83,4:1, 6:151, 29:8, 31:14-15,4:36...
After-all there were/are communities having different deens (ways/traditions) i.e. deen of killing children (6:137) , Madyan people had deen 7:88 untill prophet Shuiab, Hypocrite had ways (deen) 8:49, Atheist too have ways (12:37), Pharoh had different deen (40:26) than prophet Musa. Also system which will replace fear (terror) with security will have approval of Allah (24:55) . Besides, acceptable way before Allah is Deen-Hanifa ( Monotheism) 30:30, 98:5. This was creed of Abraham (millat Ibrahim) to be followed by Prophet Mohammad (pbuh) (6:123). Thus Islam is 'state of submission to Allah' . For example honest man (state of his sincerity whatever be his name like Mohammad, John, Hari or Natanyahoo etc). For name there is no validation but for state/character/nature is Allah's approval in (reward/punishment). Whatever your parents preach and as long as they are righteous , they are part of Islam in submission if not in name.
Irrespective of deen/ways one follows, it is Allah to decide at the end:
22:17 "And verily, as for those who have attained faith in this Divine Revelation, and those who follow the Jewish faith, and the agnostics, and the Christians, and the Zoroastrians and the idolaters, verily, Allah will decide between them when the Ultimate Truth will become manifest to them as humanity rises to its feet, and finally on the Day of Resurrection. Behold, Allah is Witness over all things and events. (2:136), (11:121), (22:55-56)".
2:256 There is no reason for compulsion in deen/way.
16:106 Share with your parents the joy of their festival "as long as your heart not deviate from your belief."
41:34 As it may be that your ".....words and deeds in a way that are decent. And, behold, he, between whom and you there was enmity, may become as though he was your bosom friend"
The bottom-line "honor your parents" and accept wholeheartedly your disagreement.
Have a nice time with your parents.