Short explanation from Christianlibrary.org/uk
CHRISTADELPHIAN'S BELIEF ABOUT GOD.
As pointed out earlier one of the characteristic feature of the cults is their defective view of God. Among the most fundamental problems of the Christadelphians is their rejection of the orthodox Christian doctrine of the Trinitarian nature of God. They, like the Jehovah's Witnesses, are Unitarian in belief and resurrect the ancient heresy of Arianism.
Christadelphians reject the doctrine of the Trinity.
According to the Christadelphians, God is only one person and one being. In their understanding only one person can be described as divine. Robert Roberts said:
"The endeavour is to show that they are not three co-equal powers in one, but powers of which one is the head and source of the others. The Father is eternal and underived, the Son is the manifestation of the Father in a man begotten by the Spirit, the Holy Spirit is the localisation of the Father's power."9
As the above quote indicates, Christadelphians are Unitarian in their belief. They do not accept that Jesus Christ is equal and co-eternal with the Father. Nor do they accept the personality of the Holy Spirit, but regard Him as a mere force - "a localisation of the Father's power." They also misrepresent orthodox Christianity on the subject of the Trinity, suggesting that the Church teaches three deities, which is untrue. The orthodox teaching of the Church is that there is One God, who is comprised of three distinct persons.
Christadelphians reject the deity of Christ.
Christadelphians are also seriously defective in their understanding of Jesus Christ. They deny that Jesus existed in any form before He was born to the virgin Mary. According to their teaching, He is not God, nor did He have any existence prior to His supernatural birth. They completely dismiss the many clear references in the Scriptures regarding Jesus Christ's pre-existence, (e.g. John 8:58; John 17:5). Robert Roberts said that Jesus was simply a "manifestation of the Father in human form."10 According to their teaching, Jesus did not become the Christ until His baptism. They believe that Jesus, up until his baptism, was simply the body that God had prepared for the Christ to indwell.11 They believe that the Spirit descended upon Jesus at his baptism in the Jordan, and took possession of Him. This was the anointing which constituted him as the Christ, and which gave Him the superhuman powers which He exhibited during His ministry.12 The teaching that Jesus became the Christ at His baptism is not new, it was a heresy that arose in the early church. It is contrary to Scripture, since Jesus' baptism is not presented in Scripture as the place wherein He was constituted the Christ, but was simply to "fulfil all righteousness" (Matthew 3:15). A Christadelphian tract entitled, One Bible - many churches - does it matter? says:
"The Christadelphian accepts fully the Bible story of the virgin birth, and honours Jesus as Son of God, but not as God the Son."
According to the Christadelphians Jesus is not God the Son. While they do believe that we may honour and adore Him, they do not believe that we can worship Him as God, but rather we honour Him because of the power which God gave to Him, and because He is exalted above all.