So first, the Trinity. Most people say they just don't understand the Trinity. Certainly the church spent hundreds of years trying to hammer out a working definition/description. After all, Christianity is descended from Judaism, which is fiercely monotheistic. Every day, the good Jew prays the Shema - Hear O Israel, the Lord is our God, the Lord is One. One God. And yet we Christians have these other two "persona," Jesus, the Risen Lord, and the Holy Spirit, the Paraclete/Comforter/Advocate in the mix. Are we polytheistic, then? Sounds like three Gods....
And so the church, using Scripture (particularly the Gospel of John) and Experience came together in great councils, particularly Nicea in 325 and Constantinople in 381to debate and refine the words and phrases we use to speak about God in Three Persons - all the same essence but not the same "personality." Sort of like water, steam and ice - all H20 but experienced three different ways (liquid, gas, solid). Except that that's not right, either; nor is the three-leafed clover example. Much ink and some blood was spilled, and many folks sent off to religious Siberia, over getting the formula wrong. Too much humanity for Jesus; not enough humanity for Jesus; Jesus as creation instead of begotten; God as OT bad guy to be succeeded or at least redeemed by Jesus as NT Love Guy. Modalism, monarchial modalism, nestorianism, marcionism, arianism.... heresies all over the place.
Most of us don't really care too much about heresies. Some of us understand that this is a mystery. Somehow we experience God in different ways, and we feel that God acts these different ways, and how that happens is not something we have to understand in all its precise glory.
Some of us today, that is. In the early centuries of the church, it was vitally important to draw some boundaries around our beliefs. The creeds (particularly the Nicene and Athanasian ones but also the Apostles') are the church's official statements of these boundaries. And within the words of the creed live deep and beautiful mysteries that many of us may spend our whole lives pondering. Certainly I believe the creeds are places where our questions often lie. They were questions in the beginning, as well; questions about the nature of God, the nature of Jesus, the nature of the Holy Spirit. Now the questions are often around the question "How much of this do I need to believe to be a Christian?" I think that's not the right question, though. Being a Christian is not all about making sure we believe the right stuff. But it does involve exploring the big topics, themes, statements, doctrines, practices. If we're doing it right, we'll be exploring them all of our days.
And as for the great commission, that's something we will explore all our days, too. Go and make disciples, and I will be with you, Jesus says to people who were pretty ill-equipped and sometimes pretty dense. Just go out there and tell the story. And God - Jesus/God/Holy Spirit - will be with you.