Peter's question exposes a lie no one else could see. Ananias brought "a certain part" of the money (Acts 5:2), yet Peter calls it a lie (Acts 5:3). Gill says Peter knew it "by divine revelation, or by a spirit of discerning, such as Elisha had, who knew what his servant Gehazi had done." The question names the real problem. This was not just about money; it was a lie told to the Holy Ghost. By asking "why," Peter also presses on how Ananias had let Satan so fill his heart. Matthew Henry says Satan "not only suggested it to him, and put it into his head, but hurried him on with resolution to do it."

Ananias gave no answer. Peter went on to say that the land had been his own and that he had "not lied unto men, but unto God" (Acts 5:4). Then "Ananias hearing these words fell down, and gave up the ghost: and great fear came on all them that heard these things" (Acts 5:5). The young men wound him up, carried him out, and buried him (Acts 5:6).

Believers in Jerusalem were selling lands and houses and laying the money "at the apostles' feet" so that no one among them lacked (Acts 4:34-35). Barnabas had just done this with his land (Acts 4:36-37). Ananias wanted to look like he had done the same.

The phrase "keep back" translates the Greek nosphizo, which Thayer defines as "to set apart or separate for oneself, i.e. to purloin, embezzle." Paul uses the same word for servants "purloining" from their masters (Titus 2:10). Thayer also notes it is used in the Greek Old Testament for Achan, who secretly took of the accursed thing (Joshua 7:1). Barnes explains that they secretly kept back a part while professedly devoting all to God.

This question shows how seriously God takes truth in His church. The Holy Spirit had just filled the believers, and they were "of one heart and of one soul" (Acts 4:31-32). Ananias tried to fake that same generosity. Peter's words show that lying to the church was lying to the Holy Ghost, and in the next verse he says it was lying "unto God" (Acts 5:4). Gill takes this as proof that the Holy Ghost is "truly and properly God."

Matthew Henry (Concise) names the root sin: Ananias and Sapphira "were ambitious of being thought eminent disciples, when they were not true disciples." The judgment that followed brought "great fear" on the whole church (Acts 5:11), and Henry adds that it would "prevent the increase of false professors." God was guarding the purity of His young church.