How can one connect the Christian belief in the Holy Trinity and the belief in the unity of God, which is so clearly witnessed in the Old Testament, without getting tied up in contradictions?"
Answer: Please read Chap. 5, III, in the book carefully once more 1. Jesus himself grew up in his people’s belief in God. This belief was characterised by monotheism, the belief in the one God, which characterises the entire Old Testament. Jesus’s disciples, of whom 12 were made apostles, were naturally also monotheists. Christians know from the writings of the New Testament that Jesus did not announce himself only as a prophet. He claimed to act in the name of God and claimed that his actions (e.g. healing, waking the dead, forgiveness of sins) made God present. Moreover, Jesus said that in him God and his kingdom had come. The disciples, that is the first Christians, recognised in the power of the Holy Spirit that Jesus’s claims were not blasphemy, did not blaspheme the one God and did not attack the true teachings of God, but were the opposite: that God himself spoke in Jesus of Nazareth, God himself was present, in other words that Jesus was the Son of God. (See: Mt 16:13-20)Step by step the disciples, that is the first Christians, realised that the unity of God was to be understood in a new and deeper way. This is what we have tried to present in Chap 5 of the book.So, to give a brief answer: Yes. The belief in the Holy Trinity does not negate the belief in the one God but deepens and differentiates it. The teachings of the Christian church present an interpretation and a further development of the teachings of the Old Testament in the light of the events of Jesus’s life (his deeds and words, his suffering, death and resurrection) as well as in the light of Jesus’s teachings, as understood by the apostles and the early Christian community by the power of the Holy Spirit.