John Calvin Commentary Acts 8:12

"But when they believed Philip preaching good tidings concerning the kingdom of God and the name of Jesus Christ, they were baptized, both men and women." — Acts 8:12 (ASV)

When they had believed. This is the miracle I spoke of: that those who were utterly amazed by Simon's illusions heard Philip. It is a miracle in that those who were foolish and dull became partakers of heavenly wisdom. Thus, they were, in a sense, brought from hell to heaven.

That baptism followed faith agrees with Christ’s institution concerning outsiders (Mark 16:47 [sic]), and those who were not yet part of the community. For it was fitting that they should be grafted into the body of the Church before they received the sign.

But the Anabaptists are too foolish when they try to use these passages to prove that infants are not to be baptized. Men and women could not be baptized without making a confession of their faith; however, they were admitted to baptism on the condition that their families might be consecrated to God, for the covenant states:

“I will be thy God, and the God of thy seed,” (Genesis 17:7).