The Psalm is probably included because of its mention of the 10 commandments, that also get mentioned in the gospel reading.
1 Corinthians 2:1-12 (13-16)
This passages expands on last week's discussion of “foolishness”. It is an example of the impenetrable logic of a believer. It goes; if you don't understand the spiritual truths it's because you are “unspiritual”. You can't receive the gifts because they are “spiritually discerned”. If you approach it with human wisdom, you won't get it. You get there by “interpreting spiritual things”.If you are attempting to talk to someone religious, it's good to figure out if this is where they are coming from early on. The conversation won't be fruitful if they are thinking that you just aren't capable of having the conversation.
Matthew 5:13-20
The gospel reading is the primary focus for today. And one of the most cited passages in all of Christianity.It starts out with the “salt of the earth”. We have some sense of what the analogy means because it has been used in our language for so long. But it's really not all that clear. It is something about being common, not of noble heritage, but also representing the best, the most worthy of praise. He goes on to say how great an example of humanity his audience is and how that is a great representation of God. There is also this thing about “losing its taste”, which seems to be an echo of the sort of Old Testament statements about sinners causing the downfall of the kingdom. But, who knows.
Next he talks about the law. Here, when it seems he has a chance to clarify who he is, what he's up to, and sort out some of the ambiguity of the Torah, he just jumbles it even more. Breaking a commandment will get you in heaven, but you will be called “least”. In other places in the Bible, breaking a commandment keeps you out of heaven. And there is no ambiguity here about the commandments still being the law but Jesus and Paul contradict that many times and the other apostles and various interlocutors are left to question it and discuss it. And of course it all has something to do with the apocalypse, if that's what “until heaven and earth pass away” means.
Finally, the statement on exceeding the righteousness of the scribes and Pharisees leads to much confusion. Jesus will go on to challenge those people and to say the sinners he hangs out with are better than them. Is this a challenge to the Pharisees who might hear these words? Or is it a wink to those who know they are the truly sinful ones, saying you need to simply be moral, not a follower of strict yet arbitrary rules, like those Pharisees are.