What does the bible mean by honouring thy father? And what if you father is not saved and a terrible sinner?
Honoring our parents may take many forms. It means obeying our parents. “Children, obey your parents in the Lord: for this is right. Honour thy father and mother; (which is the first commandment with promise;) that it may be well with thee.” (Ephesians 6:1-3) Under the Law of Moses one aspect of not showing honor was if a child should strike a parent, or even curse a parent. Such actions could even be punished by death.
You ask a very good question when you ask what if your father is an unbeliever and sinner. The command to honor your father doesn’t depend on him. It depends on you. By that I mean that the command doesn’t say to honor your honorable parents. It says to honor them, whether they seem honorable or not. Show respect and obedience, even in the face of abuse. (That doesn’t mean that one should not get out of an abusive situation as soon as possible, or that one must not report abuse.) Obey any directions of a parent that don’t directly violate one’s obedience to God. Show respect. In my generation that included saying, “yes, sir.” Show honor not because of what the parent does, but because you are supposed to show honor.
A couple of scriptures may give value to honoring even dishonorable parents, even though the scriptures are in a different context.
“Dearly beloved, avenge not yourselves, but rather give place unto wrath: for it is written, Vengeance is mine; I will repay, saith the Lord. Therefore if thine enemy hunger, feed him; if he thirst, give him drink: for in so doing thou shalt heap coals of fire on his head. Be not overcome of evil, but overcome evil with good.” (Romans 12:19-21)
“Likewise, ye wives, be in subjection to your own husbands; that, if any obey not the word, they also may without the word be won by the conversation of the wives; While they behold your chaste conversation coupled with fear.” (1 Peter 3:1-2)
While this last speaks to wives and not children, the principle is the same. If one obeys God and shows honor, that kind of life may convert even the worst of sinners just by the example. That may also be what is meant in the passage from Romans by “overcome evil with good.”