Welcome, Anesti!
The concept of the Trinity is one that's been continously argued and defined almost from the beginning--from the beginings of Christian theology, anyway, if not the faith, and has defeated minds much sharper than mine!
My understanding of the Trinity from a mainstream Christian synthesis is necessarily simplistic, but I hope accurate on the broad scale.
The three aspects of the Trinity are God the Father, God the Son, and the Holy Spirit. Although all of these three apparently did not completely manifest until the coming of the Christ, they were all present from the Beginning, and are all part of the same Divine Entity.
God the Father is God in His transcendent aspect--that part of God Who created the universe and dwells on high, all knowing and all powerful, who communicates with His followers by prayer.
God the Son is that aspect of God who became flesh in the form of Jesus, sent to be God's physical presence on Earth, to live and die as a a human being in Christ's physical nature so that He could, by assuming the suffering of all people, offer the salvation of all through the divine part of His (Christ's) nature. Jesus was both human and divine.
The Holy Spirit was sent after Jesus' death and resurrection and return to Heaven to be God's presence on Earth--the immanent aspect of God, that part of God who dwells in all who accept Christ as the way to find God.
The human/divine nature of Jesus seems to be where most of the argument has come from over the centuries. There are those who have--and do--reject any suggestion that Jesus was in any way divine, and who still consider themselves Christians. I'm not sure that there are any who deny totally the human aspect of Jesus, but it wouldn't surprise me at all, because Christians have taken positions on every point between the two extremes.
One critique of Christianity from other monotheistic faiths centers on the Trinity. How can Christians worship one God with three distinct natures? I think most Christians, if they devote much time to thinking about it, come to a point where they throw up their hands and declare it a mystery. "God is One in Three, and Three in One," which seems as good an answer to me as any.
Hindus appear to worship a vertitable multitude of gods, yet many Hindus see the Gods as different manifestations of one Divine Entity. Krishna, Vishnu, and Shiva are often pointed to as an analog of the Christian Trinity--three different faces of the same God.
My pardons to the truly theologically learned among us for my explanation, but I hope that it may help you a bit, Anesti.
