I ruminated on this one over night, and here's as simple a statement of what I believe as I can manage. First, what I would call a "belief" is something that not only I feel as a matter of faith, but can also justify solely by reason. A belief not grounded in reason or logic is one that is not likely to stand up against change over time, or personal experience. As an example of something which is a matter of faith, and not of belief - I believe in God. I'm not sure if God is a creator diety, some sort of divine being, or simply the sum total of human spirit and compassion - but I believe that God exists. I can't prove it by empirical fact, or lay out any sort of logical argument which support this faith; it's a matter of personal experience which is largely ineffable. So that's faith, not belief.
I believe in love. Not romantic love (though that has its place in the pantheon, of course) but that kind of love which is a human being's desire to see other human beings know joy and not suffer. Compassion is another way to put it, though in English "compassion" seems to be more about pity for the plight of others, rather than also the hope for good and happiness. So I call it "love" but you call it "ferdisnerd" if you wanted to - my point is, I believe that love is the greatest thing we can do in life, it is the one act which ennobles us all, and is without exception a benefit to everyone involved. Love in this sense is not selfish - it is not that one desires something from another human being, or wants them to reciprocate - love is something that we give to others and is entirely selfless. So that's why it's not strictly romantic love, which of course has a component of desire; when we love a lover we also want them to love us back, to admire us and give us affection just as we do for them. There's just not a right word in English - love is too broad. Greek has "agape" which Christian writers have appropriated to mean selfless or spiritual love, but that's not what the Greeks thought it meant, so I'm not sure it applies.
Anyway, when we love another (or ideally, all others) , we can't help but do right by them. If we seek to always do the most loving thing possible, it's unlikely we'll go far wrong. If love informs our desires and ambitions, we will probably do little harm (and especially intentional harm) and perhaps also do much good. Loving acts tend to be generous, understanding, honest, caring - acts which are not loving will be self-serving, greedy, angry, hateful or simply apathetic. Imagine a world in which all people seek always to do the most loving thing. I know that's not the world we live in, but if something is not, but ought to be, are we excused from trying to bring it about? I believe that we are not (see Kant's categorical imperative for logical defense of this..) and so ought to strive as best we can. Our faculties are limited, so we have to pick our battles. This is the battle I've picked.
But imagine if we were to do this, to lead as loving a life as we possibly could: but our love were never returned? There is still great benefit to be had to the loving individual. A loving heart is its own reward; a heart (or if you want to be strictly rational - mind) full of love has no room for acrimony, jealousy, anger, regret... a loving person goes through life experiencing the condition of love. This is a desirable condition that brings about a sense of peace, belonging, and positive affect and agency on the world. A loving person benefits not only all people everywhere (and especially those whom he has the opportunity to have not just loving thoughts or wishes - but also loving acts) - but also himself.
I believe in love. I believe in its power to redeem those who have faltered, I believe in its power to heal the hurt and refresh the weary. I believe in love as a powerful force for good - as the best thing that we can do on earth, and in our lives. It's hard, it's challenging, and we are bound not always to succeed - but I aspire to lead a loving life.
That's the one thing I can say with certainty that I believe in - that I defend with reason, that I feel in my gut, and which informs the way I lead my life. There are other matters of faith or instinct that follow, or are just inclinations or artifacts of my experience - but if I had to distill it all down into one simple statement, this is it.