At the end of Acts 4, the early church was putting into practice Jesus's radical redefinition of family from Mark 3.  The believers cared for each other as if they were blood relatives because they were members of the family of God.  In the midst of that, Ananias and Sapphira attempted a deception which strikes us a relatively minor, yet, they were struck down dead.  This came as a reminder that sin, no matter how small, is serious and it is only by God's grace that we are not likewise destroyed.