What is the ruling on burying a grandfather who was not committed to Islam, and is it permissible to attend his funeral?

If the grandfather is a recent convert to Islam and did not find anyone to teach him the rituals of his religion, he is excused for his lack of knowledge and cannot be ruled as a disbeliever for abandoning prayer, especially considering his old age and weakening intellect. If he pronounces the two testimonies (Shahadatain) and does not commit anything that nullifies them, then he is a Muslim. It is not permissible to doubt his Islam based on the possibility of him retaining his old beliefs. He might hold some erroneous beliefs for which he is excused due to his ignorance and limited religious knowledge. The imams have agreed that whoever grew up in a remote desert, far from people of knowledge and faith, or was a recent convert to Islam, and denied something from the apparent rulings, he is not ruled as a disbeliever until he knows what the Messenger brought. Sheikh Al-Islam Ibn Taymiyyah said that rulings of disbelief and discipline are not established until after the message has reached. Sheikh Ibn ‘Uthaymeen said that whoever denies the obligation of prayer disbelieves unless he is a recent convert to Islam. The questioner should invite his grandfather, teach him, and advise him gently.