The triune nature of God is possibly one of the most notable distinctives of historical Christianity, sharply separating it from all other theistic world religions (including Christian cults such as Latter-day Saints, Watchtower, etc.).  Historically, all of orthodox Christianity has maintained that while there is only one God in nature or essence, this God reveals Himself in three persons–each one being eternally distinct from the other.  Wow!  That can sound complicated and a bit difficult to get your mind around.  And I would agree that God’s nature is not a simple matter (if it were, it seems likely that the conception might be contrived or invented).  But I think there is one rational argument for God being triune or multi-personal that is compelling to just about anyone (no philosophy degree needed!).  C. S. Lewis makes a brilliantly simple observation in his book, Mere Christianity.  

Lewis talks about “love,” the most widely accepted attribute of God, and shows how this attribute in God insists upon His being multi-personal.  Just about everyone who believes in God (theists) accepts that God is love, and that this characteristic is a necessary, eternal quality of His.  This means that God did not become love at some point (e.g., after creation), but has always been a God of love.  Yet, when we think about it, love is something that is expressed between two persons.  Whether you imagine love as being simply a sentiment/feeling or will/action, it is always the expression of one person to another.  Therefore, if God is love, there must be within God at least two persons who are eternally expressing love to one another.  Otherwise, what was God prior to the creation of any sentient beings (angels, humans, etc.)?  So, as it turns out, the most widely accepted belief about God (His love) implies and suggests what might be the most contended characteristic of God (His triunity).    

Lewis also points out that what most people mean when they say, “God is love,” is that love is something that should be revered and sought after.  But this is not what Christians mean by “God is love” (thought it may be true that we should seek after and revere love).  We don’t believe that love is God, but that God is love.  There’s a huge difference!