# he was declared with power to be the Son of God

"God declared with power that Jesus Christ is the Son of God". This refers to Jesus, the Son of God. It is best to translate “Son” with the same word that your language uses to refer to a human son.

# by the resurrection from the dead

"when Jesus Christ was raised from the dead" or "by God raising him from death to life"  

# Spirit of holiness

This refers to the Holy Spirit.