How did the Lord die though we say that His divinity was not separated from His humanity even for a moment or a twinkle of an eye?
The death of the Lord Christ means the separation of His spirit from His body, not the separation of His divinity from His humanity.
Death belongs to the body – to humanity alone. It is a separation between the two elements of humanity, i.e. the spirit and the body. This does not mean that divinity was separated from humanity.
The beautiful Syrian Fraction prayed in the Holy Mass explains this fact in clear words. It says:
“His spirit was separated from His body, but His divinity has never been separated from His spirit nor from His body.”
The human spirit was separated from the human body, while the Godhead was not separated from any of them but remained united with them as before death. The only difference is that before death the Godhead was united with the spirit and the body of Christ together, whereas after death, the Godhead was united with them while each of them was apart from the other, i.e., the Godhead became united with the human spirit alone and with the human body alone.
A proof of this fact – i.e. the Godhead was united with the Lord’s human spirit during His death – is that the Lord’s spirit, being united with the Godhead, was able to open Paradise that had been closed since Adam’s sin and could go to Hades and release the righteous people of the old times who departed in hope letting all of them into Paradise with the thief who was on the Lord’s right hand on the cross and whom the Lord promised, “Today you will be with Me in Paradise.” (Luke 23:43).
The proof of the Godhead being united with the Lord’s body during His death is that the body remained completely undecayed and He could rise on the third day and come out of the closed tomb in mystery and power; the power of the Resurrection.
What happened then in the Resurrection?
In the Resurrection the Lord Christ’s human spirit united with the Godhead, was united with the body that was united with the Godhead also. The divinity never was separate from humanity neither before nor during death nor after it.