1) Either P or Q is true.
2) P is undesirable.
3) Q is true.
Many Christians do inadvertently use this fallacy to argue the existence of God. This usually goes
1) Either God exist or there is no meaning to life.
2) A life without meaning is frightening.
3) God exists.
Clearly the conclusion doesn't follow logically from the premises, but what if we change the conclusion?
1) Either P or Q is true.
2) P is undesirable.
3) Q preferable.
That's actually logically watertight for, if one thing is more undesirable than another, then the latter is preferable to the former. Of course, what we now introduce into the argument is a measure of subjectivity. Do we all find the idea of a meaningless life frightening or undesirable?
Reading the book of Ecclesiastes, we see this latter argument biblically confirmed. All aspects of life are considered "vanity". However, "vanity" in today's parlance doesn't quite give us the sense of what the original Hebrew word הֲ בֵ ל means, nor the sense that the translators of Scripture into English had in mind. Vanity is like smoke, it is something that is insubstantial. It looks real, but as soon as we reach it, it is gone. Had קֹ הֶ לֶ ת , the writer of Ecclesiastes known that our Science today tells us that atoms are mostly empty space, that the Universe is likely to end in a "heat death", he would smile and say "הֲ בֵ ל".
However, Ecclesiastes has something for the atheist as well as the theist, for the conclusion is simple. Enjoy life! Enjoy every aspect of life.
"here is nothing better for a man, than that he should eat and drink, and that he should make his soul enjoy good in his labour.(Ecclesiastes ii.24a)Of course, he then goes on to say, "This also I saw, that it was from the hand of God." (ii.24b)
By modifying the ad metum argument, we find ourselves offering some hope to those of us whose lives are afflicted by injustice. There are those consumed by anger because a suicide bomber cannot be brought to justice for killing their families and friends. What may help them is the hope that there is a justice that can be meted out beyond the confines of human life and human existence. There is the hope that a power greater than Human Justice can ensure that no-one gets away with any wrong, and yet possesses the mercy to ensure that once justice has been done, our existence continues in peace in whatever mode of existence is available after this life is over.
For those of us with faith, this is a sure and certain hope. Those of us who accept the empirical evidence for the existence of God know that, if their belief is true, then we can look forward to en end to the Evil that is within us and outside of us. The hope is a logical corollary from our faith. As St Paul says in his letter to the Ephesians:
For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Not of works, lest any man should boast. For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them. (Ephesians ii:8-10)Our faith is finite. God's grace is not. Blind adherence to the Jewish law - the "works" of which St Paul speaks - is not the source of Salvation, but only God, and we need to believe that He exists and live out that faith for that belief to do any good. But St Paul tells us one thing more. Christians were created for a purpose, just life the Jews were, namely to be instruments of God's blessing for the whole earth.
If the modified ad metum argument is to have any weight or power at all, then it requires each and every Christian to show that belief in God is better than a meaningless existence. For many people, the problem of Evil convinces them that ending their lives in meaningless is preferable to being in the presence of a god whom they see as purely judgmental, arbitrary, megalomaniac and petty, i.e. Richard Dawkins' god.
This is the mission that the Church has. We are to proclaim the Gospel to the world. It is good news that our lives are not without meaning. That the fact that we exist is for a purpose, namely to live in eternal happiness with the One Who Created us. Of course, there are agencies whose existence is purely for the silencing of this message. There are organisations of Silents just like in Doctor Who who seek only to prevent Christianity from proclaiming that life does really have a meaning and it is a meaning worth accepting.
Our moral existence is based upon the worth that we put on God and on others. God is the giver of worth, because He is the Creator, and that is why the Euthyphro Dilemma necessarily fails for Him. He is the source of Good. God is what it means to be perfectly Good. Thus God commands that which is consistent with Him. He cannot command murder for that is inconsistent with His existence.
Thus, our charge as Christians is to go out and bless the world, every single scrap of it. To glorify God in all things, in science, in society, in slums, in synods. We are to bring God into the lives of people whose existence is not happy, so that they can be happy. We are to stand against all that takes away a single iota of the worth of the least of God's Creation, for the least in God's Creation is the greatest. These are our talents given to us by God to increase and multiply true joy! If we fail to take up this calling, then it is the life of the Christian that is truly meaningless.