Does the natural law not show us our sin, discernible through our reason? I know by reason it is wrong to steal, i.e. The number "1" exists if I choose to believe it or not, or even know of it, same as "0". It is an object truth. Love is an object truth, as is God. Therefor it's opposites must be an object truths as well. Object truth can not be the exclusive property of the good. The darkness and the light both exist, both undeniable, as John's gospel tells us.
I had not intended my response to go as long as it did, but I didn't feel as though I'd do her honest question justice with a hastily-typed reply, which ultimately turned out to be full-post length:
Thanks for commenting again. The natural law certainly points toward God and helps to identify some of our sins, specifically sins which violate the natural order of creation (e.g. homosexual acts), but falls short of the complete revelation of truth given to us by Christ. Likewise our reason falls short because our wills have been weakened and our intellects darkened by sin.
With regards to good and evil as objective truths, I'm not sure what passage you were referring to in John, but the way that you worded it, "It is an object truth. Love is an object truth, as is God. Therefor it's opposites must be an object truths as well..." hints at what is known as dualism, which views good and evil as "equal and opposite" forces, with a never-ending give-and-take relationship. The dualistic argument is not in keeping with the Christian perspective that (1) evil has not always existed; and (2) evil will not always have an influence over creation. C.S. Lewis gave a good (and easy to read) discussion of the error in dualism in "Mere Christianity" Book Two, Chapter 2: The Invasion:
"There are only two views that face all the facts. One is the Christian view that this is a good world that has gone wrong, but still retains the memory of what it ought to have been. The other is the view called Dualism...the belief that there are two equal and independent powers at the back of everything, one of them good and the other bad, and that this universe is the battlefield in which they fight out an endless war."
He goes on to point out the flaws in dualism, expanding on the two points above. The most important is that evil cannot exist of its own volition. It always comes out of something good (including good desires) gone bad (by choice). Satan, who was the most powerful of angels, chose to rebel and "went bad" at the beginning of time. He was a creation of God, Lucifer, and so by definition the magnitude of his power--now, as Satan--is below and less than his creator. Jesus, the Word through which everything, including Lucifer, was created, "saw Satan fall like lightning from the sky." Jesus describes him in John 8:44, as "a murderer and a thief from the beginning, [who] has nothing to do with the truth because there is no truth in him...a liar and the father of lies."
Satan cannot create. He can only twist and distort what God has already created, through our choice of free will to join him in rebellion. Goodness--God--has existed and is omnipotent from all eternity, from outside time and creation, and will always be so.
Christ--again, Truth incarnate--"has been given all power in heaven and on earth." He, the Truth, is God, and as God is infinitely powerful. His power to create, restore, and heal, is far greater than Satan's power to twist and destroy, but that power can only take effect in our lives through our faith in him. Satan knows that his days of "roaming the earth" (1 John 5:8-9) are numbered, and will end when Christ returns in glory.