The sacrificial nature of true Christian giving.
There's a scene in Mark 12:41-44 where Jesus is sitting in a church observing different people putting money into the offering box. He watched as many rich people dropped impressive amounts of money in the box and a poor woman who gave a mere penny. Jesus then calls his disciples over to highlight something profound. He tells them that the woman who gave a penny gave not as much as, not less than, but more than the others. His point? It was that God doesn't measure your giving based on the actual amount, he measures it based on the percentage it is of what you actually have. In other words, if you give a dollar and it's half of all you have, you're giving more than the person who gives one hundred dollars which is only a tenth of all they have.
Now flip back to the book of Malichi. In chapter one, starting in verse six and going through the end of the chapter, God is calling out his people not because they weren't giving, but because of the way they were giving. Essentially, they were making sacrifices on the alter (giving) but they were offering their lame and sick animals and God was not accepting them. Why wasn't he accepting their offering? Because lame and sick animals are worthless. The people were basically giving up their trash and trying to call it a sacrifice. The point is this: A sacrifice is not a sacrifice if it doesn't truly cost you something.
We see this point even more explicitly in the second book of Samuel. The scene in chapter twenty-four is that King David has sinned and judgement is surely coming upon his people because of it. And just like God does, he provides a way for mercy to triumph over judgment. David is commanded by the Lord to, "Go up, raise an altar to the Lord on the threshing floor of Araunah the Jebusite." When David goes up, Araunah is quick to greet him with honor and inquire as to why he had come. David gives him the simple truth: "To buy the threshing floor from you, in order to build an altar to the Lord, that the plague may be averted from the people." Araunah basically responds by offering David whatever he needs, anything at all, completely free of charge. Seems like a sweet deal, right? Here's how David responds: "No, but I will buy it from you for a price. I will not offer burnt offerings to the Lord my God that cost me nothing."
These three examples from the Scripture all point to the same reality: True generosity is defined by a loss to the giver; a giving up of something valuable that you possess. And although this may sound like a bummer to some, it's actually good news because the nature of sacrificial giving isn't just to lose something but to exchange something for the sake of something more valuable. The greater the sacrifice, the more it honors God. The greater the sacrifice, the more it expresses your love for people. The greater the sacrifice, the more joyful it is for you. It was for these things that our Lord and Savior made the greatest sacrifice ever made (Romans 3:25-26, Romans 15:8-9, Romans 5:8, Hebrews 12:2). May we be emboldened to truly give as He did (John 14:12).