P symbolizes Pater because it is the first latin letter of father, the Trinity first person.
π symbolizes also father because it is the first greek letter of пάτερ
There is a latin inscription in the ring of Jaca’s Chrismon which says: "In this sculpture will recognize, reader, as follows: P designates the Father, A the begotten and the double (letter) the Begetter Spirit, the three are really one and the same Lord".
Catedral de Jaca (Huesca), Santa María de Moreruela, Santa María del Azogue (Benavente, Zamora), Calatrava la Nueva (Ciudad Real), San Martín de Frómista (Palencia), San Isidoro de León, Santa María de Poblet (Tarragona), Iglesia de la Veracruz (Segovia).
Read more in The stonemason’s signs in the Romanesque churches in Spain.