The Catechism in Questions & Answers

526. What are the prerequisites for receiving the sacrament of Holy Sealing?

The prerequisite for receiving the sacrament of Holy Sealing is belief in the triune God and the Apostles sent by Jesus Christ. The believer must also have been baptised with water in proper fashion beforehand. He must profess his faith and vow to follow Christ.

“In the proper manner” (rite): see explanation of Question 488.