hypothetically...what scriptures could be their foundation?
So this old Christian couple rents out there second fl apt to a couple which they believe to be married. In time they discover that they are not and do not intend on marrrying. What biblical stand can be presented justifying why they would evict?
None
I think the best answer is none. I don't believe this is a case of being unevenly yoked. It seems more like an opportunity to share God's love with those who don't have, know, or understand it. The idea of getting rid of people, or sending them away, because their living in sin is like saying that no one can come to Christ unless they've gotten rid of their sin first.
The "old Christian couple" have the chance, purely by 'accident', to show another couple what joy and strength there can be in a true marriage before God. And perhaps the other couple will come to see the truth and wisdom in marriage, which may lead to them coming to know Christ. Who better to show them what marriage can be than an older couple who probably have many years experience?
But what will they learn about marriage or Christ if the old couple sends them away? Nothing.