God is truly a Trinity - three persons in one. Mark 12:29 does not disprove this. It says that God is one. But we learn from other parts of the Bible that within this one there are three persons. For example, Jesus said,"I and the Father are one" (John 10:30). This shows that Jesus is fully God, and is bound with the Father in such a close and perfect relationship of love with the Holy Spirit that they are actually one.
As a result, when we worship Jesus we are worshipping God the Father. It is impossible to worship Jesus without worshipping the Father, and impossible to worship the Father properly unless you worship Jesus.
It is very important to believe in the Trinity, for two reasons. Firstly, unless Jesus is fully God, he could not have paid for our sins when he died on the Cross. Secondly, the Trinity guarantees that God is love. It is because God actually consists of three persons in loving relationship with one another that we say that he is love. It is because he is love within himself that he loves us.
Do not be put off by the Jehovah’s Witnesses. They love to take Bible passages out of context, and they even use a Bible which contains deliberate mistranslations to support their views.
The Trinity is a mystery. We humans cannot fully understand it. But that is OK, because it’s only natural that there will be some things about God that we won’t fully understand. The mystery of the Trinity is something that should make us love and trust God all the more.