I’m not saying that we shouldn’t try to be chaste. But I don’t necessarily think that premarital sex, heavy petting, etc. is always a mortal sin either. Times have changed, but our bodies haven’t. We’re only human, and holding hands for years just doesn’t seem very realistic. Though it is a good ideal, one surely to be strived for—with God’s help.
Well, firstly, nothing is -always- a mortal sin. You need to know that it’s wrong, do it anyway, and not be sorry for it afterwards in order for it to be a mortal sin. Under those circumstances, premarital sex falls within the definition of adultery in the 6th commandment.
Secondly, when you say that “times have changed,” you seem to be implying, with this statement, that morality has also changed, since the issue of mortal sin is clearly a moral issue.
I don’t see how this is possible, for the following reason.
Step 1: God’s existence provides all true morality.
If God exists, we have good reasons for thinking that moral values exist independent of whether anyone agrees with them or not.
If God does not exist, we do not have good reasons for thinking that moral values exist independent of whether anyone agrees with them or not.
However, God does exist.
Therefore, we have good reasons for thinking that moral values are independent of whether or not we agree with them.
This is because moral values of this sort (“objective” moral values,) must have their ultimate source within God.
They cannot have their source within human beings, or morality becomes the same thing as a preference for cheese; no different from any other casual opinion.
They cannot have their source within groups of human beings, because that would merely make moral values trends, and therefore, killing people in concentration camps wouldn’t really be -wrong,- just unfashionable.
In order for -anything- to be -truly- wrong, there needs to be an absolute and permanent basis for that wrongness; a basis that can only be found in the moral nature of almighty God.
Step 2: God has revealed his will for us by raising Jesus from the dead.
The resurrection of Jesus proves that Jesus was who he says he was, and therefore, that his teachings on morality (and other things) are correct. One thing that he did was to found a church, which he said would be lead to “all truth.” Because of this, we know that whatever the church teaches, officially, in matters of faith and morals, is God’s truth.
Step 3: God’s truth cannot be changed.
God created the universe. We know this because the universe is not eternal in the past, and therefore requires a cause. The only way for the same claim to -not- apply to God is for God to -be- eternal, and therefore unchanging. After all, for something to change over time involves it having a series of parts, or -moments,- through which to change. But an -actually- infinite series of parts is logically-impossible. Therefore, in order to be a sufficient first cause of the universe, God -must- be timeless. If he is timeless, his nature cannot change, and if his nature cannot change, neither can the morality based on that nature.
Conclusion: The morality revealed by God is unchanged and unchangeable, no matter how many people try to argue with it. You don’t win an argument with God, except by conceding the point.
If all that is a little much to process, just think of it this way; if God could change the nature of right and wrong, that would be like breaking his word, and an infinitely-honest being can’t -ever- break his word.