"Imagine two books lying on a table one on top of the other.... Let us call the underneath book A and the top one B.... Now let us imagine ... that both books have been in that position forever.... B's position would always have been resulting from A's position. But ... A's position wound not have existed before B's position. In other words the result does not come after the cause. Of course, results usually do ... but it is not so with all causes and results....
"God is ... three Persons while remaining one Being," writes C.S. Lewis. "As soon as I begin trying to explain how these Persons are connected I have to use words that make it sound as if one of them was there before the others. The First Person is called the Father and the Second the Son.... 'Father' suggests that He is there first—just as a human father exists before his son. But that is not so [with God].... The Son exists because the Father exists: but there never was a time before the Father [begot] the Son.... Think of the Son always ... streaming forth from the Father, like light from a lamp, or heat from a fire, or thoughts from a mind.... But ... all these ... are making it sound as if the Father and Son were two things instead of two Persons. So then after all, the New Testament picture of a Father and a Son turns out to be much more accurate than anything we try to substitute for it. That is what always happens when you go away from the words of the Bible. It is quite all right to go away from them for a moment ... to make some special point clear. But you must always go back. Naturally God knows how to describe Himself much better than we know how to describe Him. He knows that Father and Son is more like the relation between the First and Second Persons than anything else we can think of. Much the most important thing to know is ... it is a relation of love."
"The words 'God is love' have no meaning unless God contains at least two Persons. Love is something that one person has for another person.... Christians ... believe that the living, dynamic activity of love has been going on in God forever and has created everything else.... What grows out of the joint life of the Father and Son is a real Person ... the Third of the three Persons who are God. This third Person is called, in technical language, the Holy [Spirit] ... of God. Do not be worried or surprised if you find ... Him rather vaguer or more shadowy in your mind than the other two.... In the Christian life you are not usually looking at Him. He is always acting through you."
"And now, what does it matter? It matters more than anything else in the world. The whole dance, or drama, or pattern of this three-Personal life is to be played out in each one of us.... There is no other way to the happiness for which we were made. Good things as well as bad ... are caught by a kind of infection. If you want to get warm you must stand near the fire: if you want to be wet you must get into the water. If you want joy, power, peace, eternal life, you must get close to, or even into, the thing that has them.... They are a great fountain of energy and beauty spurting up at the very center of reality. If you are close to it, the spray will wet you: if you are not, you will remain dry.... The whole offer Christianity makes is this: that we can, if we let God have His way, come to share in the life of Christ. If we do, we shall then be sharing a life that was begotten, not made, which always has existed and always will exist. Christ is the Son of God. If we share in this kind of life we also shall be sons of God. We shall love the Father as He does and the Holy [Spirit] will arise in us. He came to this world and became a man in order to spread to other men the kind of life He has—by what I call 'good infection.' Every Christian is to become a little Christ. The whole purpose of becoming a Christian is simply nothing else."
Highlights from Chapter 4: Good Infection from Book 4: Beyond Personality, or First Steps in the Doctrine of the Trinity in Mere Christianity by C.S. Lewis. Click here for a clear view of how this chapter relates to the whole book.