Because the Godhead consists of the Father, Son, and the Holy Spirit. The Father is not the Son, the Son is not the Holy Spirit, and the Holy Spirit is not the Father. But these three distinct persons comprise one Spirit, God. Based on this, Jesus is the triune God as far as his Spirit. Having a fully human nature and a fully divine nature. So the fullness of God dwells in him bodily, though he speaks as a man in the person of the Son.
He was causing the universe to exist while they were killing him on the cross for our sake.