Note 3 at Acts 5:4: Ananias and Sapphira's sin was not that they kept back part of the money from the sale of their land. The sharing of their substance among the believers was totally voluntary (see note 5 at Acts 2:44, p. 584). That is what Peter was saying here. Their sin was that they agreed together to lie about the sale price so they could appear to have given all when they hadn't. We can only speculate about their reasons for doing this.

It is possible that they saw an opportunity to take advantage of the others. They could have reasoned that by making a token gesture to become a part of this communal system, they could reap all the benefits and still retain their assets. Their deception and corruptness in wanting to take advantage of others' pure, unselfish love was the sin that brought such swift judgment from God.

Note 4 at Acts 5:4: This is one of the clearest references in scripture to the Holy Ghost being God. In verse 3, Peter said that Ananias had lied unto the Holy Ghost and then in this verse he said Ananias had not lied unto men, but unto God thereby using the names God and Holy Ghost interchangeably (see note 2 at Mk. 1:10, p. 73).