Hi Jay,
To know truth requires discerning true from false, right from wrong, and a person who discerns truth must be precise in judgement, not having any bias that can compromise their better judgement, that is to corrupt and prejudice their decisions. So in that way, truth and justice are intrinsically associated to righteousness (to be "morally justifiable").
As far as truth goes, that is what we are talking about: judging right and wrong for any given case.
When you say "how can we be sure that Christianity is the truth", it's sort of a difficult question to answer so generally, because from my experience, I know that there are people who teach things that they say are Christian, but that are not true! .. and the bible itself tells us why this happens.
So that is the nature of what we are talking about .. that there are people of all religious persuasions (indeed even non-religious people), who have an interest in thinking, discussing, and knowing the truth, and the bible itself is a collection of writings of people who have been of that pursuit according to a knowledge of faith in God that originates with the Hebrew people, and that has gone worldwide over the last 2,000 years.
Jesus famously said to Pilate "You say that I am a king; it is for this purpose that I came to the world: to testify to the truth. All who are of the truth hear me".. which, when we get further into the knowledge of spirituality, we recognise that He was saying those words not as a mere man, but as a spirit that does not die: The Holy Spirit. That same Holy Spirit was there in the beginning, which is how He said John 8:58, and He is with us even until this day, which is what He promised in John 15:14-18 and Matthew 24:35.
When we come to know Him in that way, we realise that He loves and leads people in all cultures and religions, and that is how He has said in John 10:16 "I have other sheep that are not of this fold. I must lead those too, and they will listen to my voice. So they will be one one flock, with one shepherd". What He is saying by that, in more literal language, is that the sheep are common people while He is a shepherd (a king/leader), and that they are of a different "sheepfold" - that is to say, they are of a different group of people than the Hebrews He was of, in those days. He said that they would hear His voice, that is to say that He is speaking as a spirit and they would recognise Him when He speaks, so that there will come to be a united group of people from all races and religions, who have a personal relationship with Him, as a spirit.
The only thing that divides us, if we are united in spirit and following the same leader, is our individual knowledge/beliefs/culture etc.. that means to say that although I have a good knowledge of the bible while another of His sheep might have a good knowledge of Koran or Bhagavad Gita, that if we are patient in sharing with each other and not being sinful (that is pride, envy etc), then we can agree to the universal truth through coming to understand the language we each use!
.. So, you see that understanding/knowledge is more about a mind having a comprehension than the words of the scripture that has been the cause of that comprehension .. and yet, the words themselves are useful for forming comprehensions, so it follows that some scriptures (and even some translations compared to others) are particularly more useful to teaching truth than others.
It is not all God's work, and Christianity teaches that this has been the problem ever since Adam and Eve disobeyed God. The underlying sin that Eve committed in the first place was to not trust God and to not obey God, but she was tempted by the serpent: "Genesis 3:5-6 - For God knows that in the day you eat of it, your eyes will be opened and you will be like God, knowing good and evil. When the woman saw that the tree was good for food and pleasing to the eyes, and that it was desirable for obtaining wisdom, she took the fruit and ate it. She also gave some to her husband, and he ate it."
We can see through all of the rest of the scripture, there has been a never-ending fight about what is the truth about God - culminating in the major event that Jesus was crucified in the name of God, while being the only-begotten of God (born to a virgin and having authority to speak and act AS God on earth).
So the problem is that humans cannot agree to the truth. They do not love the truth enough, and it gets more serious than that, because the sin in them is what causes them to become opposed to the truth.
What you might be seeing, for example, is within yourself, you are thinking that I am not giving you everything that you need as I say this, and you are being tempted to respond to me not with gratefulness and a holy ("without sin") spirit to ask for further clarification, but with hate in order to express some anger at my failure to be perfectly what you want. (Ok, you might not be that sort of person, but there is such a type of person as that, and that is the problem: 1 John 4:18-21, 1 John 3:14-15).
Whenever people are tempted to act that way, they are not doing love to their neighbour - and it is always because of sin. (This is stuff that they should be teaching us in school!). So I understand sin according to the Seven Deadly Sins, and I understand love as "do unto others as you would have them do unto you". But the whole pursuit of Christian knowledge is understanding what love is, and what sin is, and why humans are so imperfect in love despite that they don't really need to be! .. and that pursuit of knowledge really goes beyond whatever a religion is.. it is the knowledge of the ultimate truth, itself; and there are people who love to think and talk about those matters and we can have good conversations and agree on lots of things, regardless of what our religious background is.
But not all religious people are as open-minded as that, and it is far more common to get offended and hurt by the ignorance of religious people, that causes people to retreat into groups, and guard each other, for safety so that they don't get offended by that clash of culture.. that's how there are so many religions (and denominations within them).
Have some patience when you go through
this booklet, and think about what the given verse is saying in context of the topic.. because some of the quotes are a bit of a stretch, but if you really think about it and understand what it is saying, then you will get the greater comprehension and by the end of the booklet you will understand what the problem is that makes Christianity seem to be so confusing.
Everyone has a different answer to that question. For me, there is a lot I can say about my testimony, but ultimately it is because I asked God to show me the truth and as soon as I opened my ear to hear Him, it all made sense. He has always been faithful in leading me to understand the truth that I have sought ever since, and I have such an understanding of the reality of our existence that is reliable and real, that I just know that I'm onto the real thing. But that's just me, and most people who have faith didn't actually ask God for that.
You know, most people don't actually get to see God when He is with us, although He does give us that promise in Matthew 18:20. It really is about coming to know who He is, as a spirit in our midst (John 4:23-24).
After all, St. Paul tells us plainly that, faith comes from hearing, and hearing by the word of Christ .. 
