But I speak the truth to you; it is expedient for you that I depart. For if I do not go, the Advocate will not come to you; but if I go, I will send him to you.
If your car battery is installed properly, there is a lead from the battery to the engine, and another lead from the engine back to the battery. The circuit must be complete. Then, when you turn your key in the ignition, the engine will come to life.
If we compare God to a car battery and the human race to an automobile engine, we can say that the sin of our first parents severed the connections between the divine battery and the human engine. In the Incarnation, the Son of God brought divinity down to earth, thus re-establishing the connection from the battery to the engine. In the Ascension, Our Lord brought a glorified human nature to heaven, thereby making the return connection from the engine to the battery. Once this circuit was complete, the power of the Spirit could flow. Is it any wonder that only ten days after Ascension Thursday, the Holy Spirit was given to the Church? God turned the key. The Church came to life. We have ignition!