If God is love, how do we know that's true?
God has given so much proof that He is out there, from miracles, to the truth of the old and new testament, to how they literally support the other, even though they were written at least hundreds of years apart (which requires someone who ...is not limited by time to inspire the entire bible), through its perfect truths that have no flaws, and the fact that everything that seems to hurt or upset or anger us in life, is somehow warned against in scripture, especially through Christ; the bible has all the answers to create a world of perfect peace among 6 billion+ DIFFERENT people, time after time, after time again... that is not being inspired by men, but rather, again, one who knows each of us so intimately on a level where He seems to know exactly how to keep order, peace and happiness both externally (among one another) and internally (within ourselves) for each person, time after time again. Considering we cannot create peace in our own daily lives even when we try to, it makes sense that other human beings cannot either, which means, the Church and its teachings, the bible and the Church's interpretation of the bible, can only come from a loving Creator; God, who Christ taught us about. At the end of the day, those struggle with faith can do one simple thing: sincerely ask God to help them acquire faith in Him, so that they can be sure He is truly there. I know of no one who has done this and been left unanswered/disappointed.
I'd also like to point out that in Genesis, we read "let US create man in OUR image"... the fact that the Trinity consists of the love between God the Father + God the Son = the Holy Spirit, and how human beings are created through the love... between Father + Mother which = their children/their child... and, of course, recognizing that the Church's teachings on this revelation came thousands of years after Genesis was written... is enough to reveal to us that Christ was, is and always will be trustworthy, and so is His Church, who is being run not by men, but men who are vessels of God's Divine Providence.
God is love because God is a union of "Father, Son and Holy Spirit" = the love between Father & Son is the Holy Spirit, which means, God is love itself, through the three unique persons in the Blessed Trinity, which is who God is. I should ...mention that I'm not speaking on my own wisdom, obviously (I couldn't make this up if I tried :-)), but between the Pope's book titled "God is love" and many other sources like the Catechism, this is all explained, and it is all rational, and it simply just couldn't be inspired by anyone except for a being unlimited by time: our loving God.
God revealed that "the law" (that is, conduct that creates love, instead of heartache, and hate, jealousy etc.) is written on the "human heart" - in other words, we are created in such a way where deep down inside, although we are easily fooled, we know right from wrong, though it helps to have a model to see it lived out and it helps to have someone explain it further to us (The same way the law of the US has legal commentary to explain a simple law for us, and how there is case law, which are real life people with real life situation that the law is applied to for us to further understand how the law is to be lived out in order to spread peace.
Christ is the equivalent to a "legal commentator" ... on God the Father's law. Christ explains it for us, and we quickly see that peace is what would result if we choose to cooperate with God and follow that law.... and (2) he is also "real life case law" in that Christ followed God's law, which is truly "love all people, no matter what they've done, and no matter who they are) perfectly. Without these things, it is difficult to discern good and bad without being misled or confused.
Considering Catholic beliefs are rooted in the fact that this life is a spiritual battle between good v. evil, it makes sense that because everything the Church teaches tends to eliminate everything that we enjoy doing, but that proves to cause some sort of pain or negative emotions down the line, it makes sense that following its teachings and using the Sacraments (Which are sources of grace that we can choose to use with our free will, in order to receive the strength not to engage in conduct we enjoy doing (from pre-marital sex, to gossiping, to being spiteful, to seeking revenge, etc.), will leave to an all loving God who is not the cause of such negative emotions in us - the opposite of a loving God, would be that influence (Which, again, Christ, and thus, His Church, and Christianity as a whole, recognize as "the enemy" (Satan; the one who hates God, and thus, hates God's creation and God's children; every human being in creation)