Baptism
John 3:5
Amen, amen I say to thee, unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God.
Context of the passage
Jesus instructs Nicodemus, a Pharisee seeking understanding, on the necessity of spiritual rebirth to enter God's kingdom.
Catholic reading
Jesus' pairing of 'water and the Spirit' as the necessary means of rebirth is understood by the Church as the institution of the sacrament of Baptism, an actual instrumental cause of regeneration, not merely a symbol of an already-accomplished spiritual reality.
Common objection
'Water' here refers to natural, physical birth, and 'Spirit' to spiritual rebirth by faith, so this verse does not teach sacramental baptismal regeneration.
Response
Nicodemus already understood natural birth and asked how a grown man could be 'born again,' so Jesus is not repeating that concept; the single phrase 'water and the Spirit' names one new birth, echoing early Christian and patristic understanding that this refers to the water of Baptism together with the Spirit's regenerating action.
The Church Fathers
“They then receive the washing with water... for the remission of sins that are past... this washing is called illumination.”