Sign of the Cross

In nomine Patris, et Filii, et Spiritus Sancti. Amen.

In nomine Patris,
In >> in.  "In the name of the law." Nomen >> nominate, nominal, nomenclature. Pater >> patrimony, patriarch, paternal. The endings of Latin words tell how the words are used, though the same use might be indicated by different endings, and the same endings may indicate different uses. Compare "he" and "him", "she" and "her", "father" and "father's." Latin doesn't use "the." In [the] name [of the] Father.

et Filii,
Et as in etc. [et cetera], et al., Et tu, Brute?  Filius/filia >> filial. And [of the] Son.

et Spiritus Sancti. 
Esprit de corps, as the spirit moves you, in high spirits. Sanctify, sanctuary, saint. And [of the] Holy Spirit.

Amen.
[from Hebrew] >> Amen.