How do we know that God didn't lie at Sinai? Maybe he said he was good and compassionate, and promised us reward, but it's all a trick? Why believe that God is true?
In context we know that God is good from observing His deeds, i.e. that He created human beings as an act of pure benevolence, not because He needs anything from us as the shaar bitachon (gate 4 of Chovos Halevavos) says.
That one is conscious of G-d's abundant goodness to man, and how He brought him into existence out of abundant and pure benevolence and kindness, without man being worthy of this, nor because G-d has any need for him, but only out of generosity, benevolence, and goodness, as we explained in the gate of reflection of this book, and like King David said "Many, O L-ord my G-d, are Your wonderful works which You have done, and Your thoughts which are toward us: they cannot be reckoned up in order to You; if I would declare and speak of them, they are more than can be numbered" (Tehilim 40:5)
and likewise in the shaar avoda Elokim (Gate 3 ibid): "The first is the universal goodness of G-d which embraces all mankind, in having brought human creatures into existence when previously they were naught; in keeping them in life and bestowing on them bounties which we have cited in the second treatise of this work. They are accordingly under a universal obligation of service to the blessed Creator."
I truly believe that when one goes astray from the direct view of G-d (via Torah), (kol ha'adam kozev) Each of us are only human and have made a mistake. It appears as if G-d is hiding or lying. We become blind in the lies of our limitations.
Barukh HaShem, He took the initiative and compassion to offer us His free education (Torah) to help us channel out lies from the authentic.
Many learnt that the word lie "sheker", each letter stands on only one leg, unstable.
Many learnt that "Emeth" truth stands on 2 legs. Strong foundation.
A lie will eventually cause a breakdown of one's recognition of the many blessings of G-d.
When we are not sure concerning spiritual blindness or deceit, "Dirshuni vechiyu" HaShem says "seek sincerely for me that you may live. We should use our brain, and of course seek a learned person who is also full of ahavas Yisra'el for guidance to broaden our horizons of how to attain this wisdom.
G-d is good, yes. Evil is the absence of productivity. What may appear to be evil actions from God is the balance of positive and negative. Even destruction and recycling of nature (the universe) follows G-d's good laws.
It is not fair for one to judge G-d. We cannot comprehend beyond the 3rd or 4th dimension.
I love the expressions of "derekh HaShem," Huqim, ofan, orach" all variations to deep meaning of "way, path, law, etc., which are not limited to a human concept of a celestial king sitting on a throne, but goes even deeper in meaning than that, like the throne of all the laws of Physics, Ethics, Justice, etc.
Our brain interfaces with over 110 billion neuron receptors, very complex. Imagine them tuned into the Torah mega battery of Reality.
Truth or lie does not depend on our opinion, experience, or belief. Truth is independent. We either hook up to it, or aimlessly wonder. A little knowledge of Truth is better than none, but we must pursue further so as not to become stagnate.
I assume you're referring to Shemot 34:6-7
Since we're playing devil's advocate: There are multiple places in Chumash where God is not "compassionate," "slow to anger," or "forgiving," even after He said He would be:
Well, God is known for playing tricks.
If you want a source within the Torah that God didn't lie, see Deuteronomy 32:4.