Christians start and finish each and every of their work with the sign of Cross, because the Incarnate Son of God has made transparent the unselfish Love of the Holy Trinity — towards Mankind — through His Cricifixion.
By making this Sign we manifest our response to the self-denying sacrificial Love of God.
A solitary and insensitive God of Jewish and Muslim theology suffers no Cross, neither enjoys the Resurrection. However, such a lonely God, »only there upstairs«, would not conde’nscend to brotherly love with men — »here downstairs« — let alone to seal His infinite Love with His death on the Cross.
That is the reason why St. Paul did not shun »the shame of Christ« and has boasted only in the Crucified Lord.