Here are some of the quotations he cites.
“What is water without the cross of Christ?” Ambrose asks his newly baptized, and answers, “an ordinary element.”
A post-Augustinian sermon: “Through the sign of the cross you are conceived in the womb of your holy mother, the Church.” Rahner says, “It is only by the procreative power of the cross that the church is fructified.”
He explains that ultimately the symbolism is based on Romans, with its relation of baptism to the cross. “Baptism is “the mystery of the wood in the water.’” (P. Lundberg)
“Jesus Christ was born and was baptized, so that he might sanctify the water by his passion.” (Ignatius of Antioch)
“This cross symbolizes the fact that the baptismal water has through the death of Christ been made a bestower of life—it is the tree of life." Then he talks about a wooden cross that was put up in the Jordan river—in many Eastern liturgies the baptismal font is actually called “Jordan”, and a wooden cross is dipped in the water at the consecration [of baptism].
Rahner explains that the Paschal candle is a symbol of the cross of Christ.
But a new element is that the wooden cross is now a giver of light.
The same fire bursts forth from it that was associated with Jesus’ baptism.
The lighted candle is a symbol of the cross of Christ, the light of the world.
The cross is also a bringer of light, and when men seek to express this mystery in explicit liturgical form, they do so by lowering a burning candle into the baptismal font as a sign that, by the power of the cross, the water is a source of the lux perpetua, the everlasting life of light. In a word the cross is both the tree of life and the light bringer and both symbols represent Christ himself who “by his Passion sanctified the water” by giving to it the doxa, the glory which he had won upon the cross, the power of the Holy Ghost.
In thinking about how the mystery of Jesus' baptism relates to that of the cross, I was reminded of the beautiful antiphon from the feast of Epiphany:
“Three mysteries mark this holy day:
Today, the star leads the magi to the infant Christ;
Today, water is changed into wine for the wedding feast,
Today, Christ wills to be baptized by John in the Jordan River
To bring us salvation.”
And in another form:
“Today, the Bridegroom claims his Bride, the Church, since Christ has washed her sins away in Jordan’s waters, the Magi hasten with their gifts to the royal wedding, and the wedding guests rejoice, for Christ has changed water into wine. Alleluia!”
On Epiphany, the liturgy links the mystery of Christ's baptism with that of the revelation of Christ to the nations (represented by the Magi) and the miracle at Cana. I've been trying to think of how those two mysteries are also related in some way to the cross. I think it could be a fruitful area of reflection. But that will be a post for another day.