This bridge is a metaphor for Christ, and there are three stairs, which become the three steps toward deeper union with God. The first stair, she says, is the affections, which are symbolized by the feet (she says "the affections carry the soul," much like our feet carry our bodies).
The second step is the "stair by which you can climb into his side, where you will see revealed his inmost heart." (Do you hear the echos of early Sacred Heart spirituality here?)
The third step is where the soul reaches Christ's mouth, "where she [the soul] finds peace from the terrible war she has had to wage because of her sins."
So, Catherine continues, "At the first stair, lifting the feet of her affections from the earth, she stripped herself of sin. At the second she dressed herself in love for virtue. And at the third she tasted peace." The bridge itself, with these three steps, was raised up by the cross of Christ, a link (bridge!) between humanity and God's divinity.
And, as as side note, those three steps correspond to the classical spiritual path of purgation -- illumination -- union that is found in so many other writings!
LOVE is the central element to this cross, and (Catherine tells us) we human beings are drawn by love: "You can hardly resist being drawn by love, then, unless you foolishly refuse to be drawn."
Ah! What lovely imagery! We are drawn to God by the love God offers us, and the only way we are not drawn is if we hold back (and who would do that?).
Throughout teaching about the history of Christian spirituality, what is most striking is the emphasis on love, the idea that we know God only by loving. We can never comprehend God with our minds, but with our desire and ability to love, we can know God deeply.
Hear, O Israel! The LORD is our God, the LORD alone!
Therefore, you shall love the LORD, your God,
with your whole heart, and with your whole being,
and with your whole strength.