Since the Bible emphatically states that there is only ONE God, why do all orthodox Christian churches teach that Jesus is God?  How can anyone believe Jesus is God in the face of so many Bible passages saying (seemingly) there can be only one?  Didn't Jesus pray to God?

This is the origin of the doctrine of the Trinity.  There cannot be three separate Gods.  Nor, is there one God sometimes acting out the part of Jesus, sometimes assuming the role of Holy Spirit, but usually playing our Heavenly Father. But while the Bible clearly states there is but one true God; it just as clearly demonstrates that Jesus is God, that the Holy Spirit is God, and simultaneously our Father in heaven is also God!

This is the doctrine of the trinity:

Within the unity of the one true God are three distinct Persons -- all co-equal in nature, attributes, and eternity.  Jesus is not the Father, and the Father is not Jesus. The Holy Spirit is not the Father; the Father is not the Holy Spirit. Jesus is not the Holy Spirit and the Holy Spirit is not Jesus. Yet, all three persons (plural) are the one (singular) true God.

When the Bible teaches there is only ONE God, Christians believe it because we trust the Bible.  When the Bible says Jesus is God, Christians believe that also for exactly the same reason.  When the Bible says the Holy Spirit is God, Christians must likewise accept this because the Bible doesn't lie  How can we reconcile these three beliefs with the seemingly contradictory belief that there is only ONE God?  Only because the Bible says so!  We trust Scripture and accept it although we can't fully understanding it.  We devise a word ("trinity") to label this concept we don’t really comprehend, and we go on from there.

Our finite minds cannot fully comprehend anything about an infinite God.  We know what a long time is, but we cannot fathom everlasting life.  We do not understand a God without beginning; how could he have existed forever in the past, and forever in the future at the same time?  We cannot conceive of a being totally independent of time and space—eternal, unchanging, omnipresent, transcendent but yet personal and immediate, and totally self-sufficient and complete within himself--perfect.  We can’t!  Why reject this attribute of God (his triune nature) just because we cannot understand it either?