Many dear souls think that they have been born of God because they have joined the church and have been baptized. Every saved person ought to join the church and be baptized. But joining a church and being baptized does not make one a Christian.
This is another short but to-the-point article that discusses, as the title obviously reveals, the necessity of being born again. In short, the author shows that being born again isn't something that comes about by something a person thinks nor something a person does, but rather it is a work of the Holy Spirit; sudden, silent and sovereign.
Read this article here: How We May Know We Are Born of God
For later reading, you can find this article in "Calvinism and the Reformed Faith" and in the "Soteriology" section.
In His service and grace,