All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The best way for the children to be kind and dutiful to their parents is to guide them to righteousness and clarify to them the truth when they commit a mistake. We have a good example in the Prophet Ibraaheem (Abraham) with his father as he advised him in a good and nice manner as mentioned in chapter Maryam [Quran 19].
If what you mentioned about your father is confirmed, that he said ''The Injeel is not the Word of Allaah'', then his statement has two probabilities:
1- He might mean that the Injeel which we have today was forged and falsified and that it is not correct to describe it as being the Word of Allaah; if this is what he meant, then he is correct.
2- If he meant that the original Injeel which was revealed by Allaah to 'Eesaa (Jesus) is not the Word of Allaah, then this is disbelief.
Similarly, his statement ''The revelation did not stop after the Prophet '' includes two probabilities:
1- If he meant some kinds of revelation, like insight, dreams which come true, then he is correct; as these kinds still exist.
2- If he meant the revelation which used to come down from the heavens and which ended with the death of the Prophet then this is also disbelief, because Allaah ended the revelation with the death of the Prophet . For more benefit, please refer to Fataawa 88413, 87355 and 82053.
As regards the congregational prayer, then the scholars differed in opinion about its ruling but the most preponderant opinion is that it is an obligation, and if he abandons it without a sound excuse, he is sinful.
Slaughtering in Adh-Ha 'Eed is a matter on which the scholars differed in opinion as well; the preponderant opinion is that it is a recommended Sunnah and this is the view of the majority of the scholars . Some of them stated that it is disliked to abandon it if one is able to do it.
Therefore, we advise you to be patient with your father and be kind to him as this will help you in turning him into a righteous man. You should seek the help of the scholars or knowledgeable people in your country to clarify to him his mistakes, [if any] and you should not forget to earnestly supplicate Allaah to guide him and enable him to repent.
Finally, it should be noted that you are obliged to accompany him with appropriate kindness despite the sins he committed even if these sins take him out of the fold of Islam.Allaah Knows best.