If you read Acts 5, you'll come across the story of Ananais and Sapphira.
Basically, they sell some land to give money to God's work. However, they say that they are going to give all of it, pretend they have then they die. On the spot.
Every time I've read this, I thought that it seems pretty harsh. Yeah, they didn't quite tell the truth, but they die because of it - why were Ananais and Sapphira so severely punished?
Verse 3 gives us a clue as Peter asks, 'why has Satan filled your heart to lie?'.
The issue isn't about money. It's about the condition if their heart. This couple hadn't been asked to give anything, what they did was voluntary. What they intended to do - give money - was a good motive, so what went wrong?
Ananais and Sapphira had agreed to lie. They agreed together that they would say they'd given everything when they hadn't. They could of said they were giving some of the money, but they didn't, they pretended to give all of it. It's like they wanted others to look on them as sacrificial, generous all-out-for-God people. They decided they were going to keep a certain amount for themselves, which really is fair enough - it's their stuff anyway. However, it's the deliberate deception that is the problem. Peter says Satan had filled their heart. It's not a small matter, they had decided to look good in front of man, little considering what that would look like in God's eyes.
Peter called out the sin as they brought their gift. God had given him knowledge of their deception, and he called it out. If he hadn't then it could of enticed others to do the same - to deceive each other. Peter was concerned that satan had influenced their hearts and that cannot be allowed in a community of God-fearing people. It's a serious business. Serious enough to die by God's standard.
This couple chose to lie to God and man, and they were punished for it.
It must of been a terrifying lesson for all the believers. I mean, I know that I have lied at times, I have not been 100% truthful, and I thank God I'm still alive! If anything this story serves to show us sin is serious. If we allow it to grab hold of us then we are no better off than these two. Even if we don't die on the spot, sin, deception, wilful wrongdoing cause separation between us, others and God. This is the serious truth.
Next time Satan tempts you to allow him to rule your heart, think about Ananais and Sapphira. It didn't end well for them, it went for you either.