Hey! I think that when we talk about the truth of something we refer to the only one and unique truth because if there were many truths it wouldn’t be truth anymore, right? :D So this is why we have to use the definite article the which is used to specify something.
Another example is “the sun” because there is only one sun.
In the case of “a lie” is different because there is not an specific lie, but there are usually many lies, so a lie can be one among many.
I hope I have not confused you more, and that this can help you. :)
In English, nouns can be countable or uncountable, or they can be both with a difference in meaning for each type. ‘Lie’ is countable, while ‘truth’ is an uncountable noun. ‘Truth’ can be countable in certain English phrases, such as ‘home truths’. For example: The boss told him a few home truths about his shoddy work, and she wanted him to improve his work performance over the next few weeks.