One of my favorite arguments for Christianity is the moral argument. It's a particularly strong one because, to quote William Lane Craig, "it hits people where they live". When a man realises they need God to justify objective morality, there are usually only two responses: Conversion, or reprobation.
I recently came across a post on this topic, shown by the image below:
I'd like to tweak this argument a little. As Scripture says, "...He makes His sun rise on the evil and on the good, and sends rain on the just and on the unjust." (Matthew 5:45). Therefore, a secularist can justify marriage, because by instinct and practice, he can receive its benefits without giving God the glory due to Him. What a secularist cannot justify is goodness itself, much less calling other models "evil".
As C.S. Lewis argued, a man cannot call a line crooked unless he has a concept of what a straight line is. We can show, objectively, that marriage has certain effects, both in society, and in our personal lives. Thus, we can, and very often do, make secular arguments for marriage, and against other models. We can show that it provides economic stability, reduces the spread of disease, and generally gives any children that result a more fruitful life. But what a secularist cannot say, without God, is that these things are necessarily good.
Indeed, the very reason we rebel against God in the first place, even for some of us resulting in the delusional denial of His existence, is that with Him out of the way, we get to define "good". What you define as good, someone else may define as evil. That is why, just as there are secular arguments for marriage, there are secular arguments against it.
But here's where it gets difficult for the secularist. When you pit the secular against the secular, the secular wins. You bring a secular argument for marriage, it will be met with a secular argument against marriage, and the result is the chaos you see in today's secular society. Secularists lack foundation for establishing a goal, and so especially in cases where both arguments work, neither side ultimately wins... except through force. You want to make a secular argument for marriage? Well, now you're just a homophobic dinosaur clinging to religious nonsense for which we no longer have need. So now you get fined for "hate speech", which may increase to "re-education", and when that becomes normalised, you may even end up in prison.
But a Christian is capable of presenting both a Biblical and secular argument for marriage. We can show that marriage works from a secular perspective because we do not live in a world where God does not exist. Because God exists, His commands, which are very much not arbitrary, have practical applications, and beneficial results. We could simply say that marriage is good because God declares it so. But we can show that marriage is good because God has made it so. Thus, while secularists are stuck pitting Satan against Satan, creating their divided houses which collapse into the very sand on which they are built, we, as Christians, are able to show not only that there is a standard of good, but also... we fall short of it.
Can any secularist, even by his own standard, say that he is good? In his corruption, of course he can. He is not holy enough to shun his own sin! He can tolerate deviation, he can celebrate a little darkness, he can drink from a dirty glass. But as a little leaven spreads throughout the lump, so a little sin makes a sinner. What do we call a man who murders just once? A murderer. What do we call a man who cheats once? An adulterer. And what does James say? "For whoever shall keep the whole law, and yet stumble in one point, he is guilty of all. For He who said, “Do not commit adultery,” also said, “Do not murder.” Now if you do not commit adultery, but you do murder, you have become a transgressor of the law." (James 2:10-11).
And transgressors we are! For indeed, the very reason we have ousted God, even to the point of denying His existence, is precisely so we may rebel! We are not good people. But though we may prefer the secular, ultimately, our lives are not so trivial as to be forgotten about upon their end. This is the flaw of the secular. We may bicker about our goals for our lives, because we erroneously act as if this is the only one we have. So what if we prefer fornication? If we believe we merely die and rot, we might as well. But as Scripture tells us, "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge." (Hebrews 13:4). So wait, it's not just a case of cause and effect? God will actually judge me for what I do?
Well now it's easier to take a side. Clearly, marriage is the only legitimate context for sex. Not just because it leads to good things, benefiting both individuals and society as a whole, though that is a nice plus. No, now it's a matter of everlasting life and death. But wait, now what do we do about our shortcomings? If fornicators and adulterers God will judge, what do we do as fornicators and adulterers? Are we just boned? We did the crime, now we must do the time? NO! Praise God, there is a better way! "For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him." (2 Corinthians 5:21).
Jesus never fornicated. Jesus never committed adultery. Jesus never sinned. Not even once. Yet, on our behalf, He faced the wrath of God, so that all who believe in Him will be saved. You fornicated, you believe in Christ, He effectively becomes the fornicator. You become the one who walked in God's ways all the days of your life. If not? "“He who believes in Him is not condemned; but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God. And this is the condemnation, that the light has come into the world, and men loved darkness rather than light, because their deeds were evil. For everyone practicing evil hates the light and does not come to the light, lest his deeds should be exposed. But he who does the truth comes to the light, that his deeds may be clearly seen, that they have been done in God.”" (John 3:18-21).
You see, then, that a secularist can justify marriage, but he cannot justify himself. It is Christ who justifies, and a secularist tries to separate himself from Christ. The result is his just condemnation. You did the crimes. You cannot do the time. But without Christ, you will do the time. Therefore, seize the time, for today is the day of salvation. Repent, believe, and receive the salvation that can never be found in anyone but Christ.