From The Gospel Coalition-
Historically, the church’s ministry of grounding new believers in the rudiments of Christianity has been known as catechesis. It is a ministry that has waxed and waned through the centuries. It flourished between the second and fifth centuries in the ancient church. Those who became Christians often moved into the faith from radically different backgrounds and worldviews. The churches rightly took such conversions seriously and sought to ensure that these life-revolutions were processed carefully, prayerfully, and intentionally, with thorough understanding at each stage.
With the tightening of the alignment between church and state in the West, combined with the effect of the Dark Ages, the ministry of catechesis floundered in large measure for much of the next millennium. The line between natural and spiritual birth virtually disappeared. According to the centuries-old practice, infants baptized into the church were, in theory, to be catechized later in the faith. But too often nothing of the sort occurred. As a consequence of such neglect, great numbers of persons who claimed to belong to Christ had little idea of what that might even mean.