First, Augustine observed that crucifixion is not only painful, it is painful and public. The public nature of Christ's death inspires me to face death heroically.
Second, Augustine observed that since Adam brought death through a tree, it was fitting that the New Adam destroy death by hanging on a tree.
Third, John Chrysostom and Theophylact observed that by being lifted up on the cross, Christ sanctified the air.
Fourth, Athanasius observed that by being lifted up on the cross, Christ shows that He has prepared the ascent into Heaven.
Fifth, Gregory of Nyssa observed that the shape of the cross was fitting for because it extends in the four directions and is therefore universal. Also, Athanasius wrote that the one outstretched arm sanctified the those in the past and the other arm as outstretched to the future. So we have both a spacial and temporal universality signified in the crucifixion.
Sixth, Augustine says the parts of the cross signifies the following:
- Breadth – This pertains to Christ’s hands and thus "good works"
- Length – This pertains to the upright nature of a tree and thus "longanimity".
- Height – This pertains to the top and Christ’s head and “the good hope” of the faithful.
- Base – The base is the root and it is hidden, thus it signifies “grace”.
I adapted these seven reasons for the wooden cross of Christ from Saint Thomas Aquinas III q. 46, a. 4.