Everything Jesus says is true. I didn't say I don't know the Truth but there's no way you or I or anyone else can know the truth of everything and in many things, there's no way we can know that what we "know" is actually corresponding with reality. If you and I don't agree on the nature of Hell, would you say I'm lying because we differ? (I don't know that we do just using that as an example). People differ on many things. I hold to a different view of salvation. My different view doesn't make me a liar. I could be wrong but I also could be right.
I don't doubt there are a lot of things not known, but when something is presented there is no doubt that you can know if it is true or false.
The issue comes when a person doesn't want to listen to the Spirit. If the Spirit of God lives in you(and He does in everyone who is in Christ) then He is not going to just sit back and make you wonder if what you are hearing is true or false.
Sure, there are times when we do not want to listen. But that's what discipline is for. If you are trained by it, then you will be able to discern.
If you hold to a view, and teach it, and it is false - then yes you are a liar. That is the truth. The kingdom of God is not a free for all.
Thinking you know something is vastly different than saying you know something absolute. The heart of the matter is this - if you want to know the truth you absolutely can. And if God does not want you to know something, then He will tell you. It's really not anymore complicated than that.
John 16
7 If you abide in me, and my words abide in you, ask whatever you wish, and it will be done for you.
8 By this my Father is glorified, that you bear much fruit and so prove to be my disciples.
James 4:3
You ask and do not receive, because you ask wrongly, to spend it on your passions.
Matthew 7
7 "Ask, and it will be given to you; seek, and you will find; knock, and it will be opened to you.
8 For everyone who asks receives, and the one who seeks finds, and to the one who knocks it will be opened.
9 Or which one of you, if his son asks him for bread, will give him a stone?
10 Or if he asks for a fish, will give him a serpent?
11 If you then, who are evil, know how to give good gifts to your children, how much more will your Father who is in heaven give good things to those who ask him!
Jesus is speaking the truth. You can know if something is true or not. Just ask. It's not about knowing all things, just about knowing if something is true or not. In Christ there is no doubt.
James 1
5 If any of you lacks wisdom, let him ask God, who gives generously to all without reproach, and it will be given him.
6 But let him ask in faith, with no doubting, for the one who doubts is like a wave of the sea that is driven and tossed by the wind.
7 For that person must not suppose that he will receive anything from the Lord;
8 he is a double-minded man, unstable in all his ways.