The gospel is free

Ps 50 9 I have no need of a bull from your stall or of goats from your pens,

10 for every animal of the forest is mine, and the cattle on a thousand hills.

11 I know every bird in the mountains, and the creatures of the field are mine.

12 If I were hungry I would not tell you, for the world is mine, and all that is in it.

13 Do I eat the flesh of bulls or drink the blood of goats?

14 Sacrifice thank offerings to God, fulfill your vows to the Most High, 

This Psalm is teaching that God is always making judgements about the religious and political systems. God who goes out from Israel as a fire and a tempest to fight Israels battles is the same God who blesses His people and curses the wicked. This phrase "19 You use your mouth for evil and harness your tongue to deceit. 20 You speak continually against your brother and slander your own mother's son." This is a phrase that is used as a description of the wicked who belong to the nations who are Israels enemies. But in this case the wicked are part of Gods people within the nation of Israel. 

These wicked people are using the sacrifices of God in order to appease God. The Psalms teach there is nothing that we can bring to God that will make us acceptable. We believe that God has made a covenant with us as the basis of our salvation. The covenant of grace is one sided. 14 Sacrifice thank offerings to God, fulfill your vows to the Most High, 15 and call upon me in the day of trouble; I will deliver you, and you will honor me." I believe the Psalmist is being sarcastic when He says to "sacrifice your thank offering to God." The Psalmist is saying that God doesnt need anything from us but a thankful heart. He is using the word "sacrifice" in the sense that these people are centered on the insignificant things in being acceptable to God. He really is saying so your always focused on the physical things in the sacrifice ,but your not focusing on the thankful heart and this is more important than the sacrifice. You want to practice the sacrifice then practice being thankful!

God is declaring here that He alone has done the work in order for Israel to be free both political and religious. You will find that this struggle between the solas and placing something or someone in the place of God has gone on since the beginning of time. This Psalm enforces the idea that salvation is in God alone by showing what this God is like....The One who judges His people. He comes as Israels army and fights their battles, He doesnt need the religious things to be pleased with His people, these things do not give life. God doesnt need to be appeased to feed His people. God  wants a thankful heart as a return for His works. Spending our energy in thankfulness to God is not a work.

This Phrase "call upon me in the day of trouble" is always used by the Psalmist as he teaches that God has saved His people through putting Himself on display. So when the Psalmist teaches the character of the thankful heart , he is saying that this petition is the evidence that Gods people understand that this covenant is one-sided. The wicked turn grace upside down and use people and righteous acts to be acceptable to God as a display that they are trying to add to grace. This phrase "call upon me" is presupposing that we can bring nothing and we need everything from God. 

Why is this phrase so important? Because when we give religious things and people the power to be a god we destroy the freedom that is in the gospel. We bring the gospel down so that man can control God. This is why God teaches that He alone has the power over life and death. He alone enacts His judgement over the nations. He alone acts to secure Israel and to provide all that they need in order to prosper.  The gospel is either so free that it strikes against our sense of what is owed or it is a curse that weighs us down and destroys us. We cannot think of grace as pragmatic. This is what this Psalm is teaching.