Thomas Schreiner, Galatians (Zondervan, 2010), 392:

The cross plays a bookends role in the letter [of Galatians], for just as Paul begins the letter by featuring the freedom won in the cross, so too he closes the letter by underlining the significance of the cross.

Paul’s only boast is in Christ’s cross, by which he is crucified to the world and the world is crucified to him (6:14). The cross and eschatology are inseparable. Just as the cross liberated believers from the present evil age (1:4), so too it crucifies attachment to this world (6:14). The opponents boasted in circumcising converts and took pleasure in external accomplishments because they lived to win the applause of others (6:12–13). They lived for comfort in order to avoid persecution.

The cross severs a love affair with the world and grants a person (by grace!) a desire to boast only in the cross. A new reality—a new age—has begun through the cross, and Paul summons the Galatians and all believers to find their joy only in the cross and to renounce any boasting in human accomplishments.