35 Beware lest ever like Simon thou come to the dispensers of Baptism in hypocrisy, your heart the while not seeking the truth. It is ours to protest, but it is yours to secure yourself. If you stand in faith, blessed are you; if you have fallen in unbelief, from this day forward cast away your unbelief, and receive full assurance. For, at the season of baptism, when you have come before the Bishops, or Presbyters, or Deacons —(for its grace is everywhere, in villages and in cities, on them of low as on them of high degree, on bondsmen and on freemen, for this grace is not of men, but the gift is from God through men,)— approach the Minister of Baptism, but approaching, think not of the face of him you see, but remember this Holy Ghost of whom we are now speaking. For He is present in readiness to seal your soul, and He shall give you that Seal at which evil spirits tremble, a heavenly and sacred seal, as also it is written, In whom also ye believed, and were sealed with the Holy Spirit of promise.
Source: Catechetical Lectures (New Advent)