Ichthys is a Greek acronym, an early symbol of Christianity. The Wikipedia entry defines it as:
ΙΧΘΥΣ, or also ΙΧΘΥC with lunate sigma (Ichthys) is a backronym/acrostic for "Ἰησοῦς Χριστός, Θεοῦ Υἱός, Σωτήρ", (Iēsous Christos, Theou Yios, Sōtēr) contemporary Koine [ie̝ˈsus kʰrisˈtos tʰeˈu (h)yˈjos soˈte̝r], which translates into English as "Jesus Christ, Son of God, Saviour."
My Latin translation into Classic Latin would be:
Iesu Christi Filii Dei Salvatore
Thus, the acronym equivalent to ICHTHYS would be ICFDS? or maybe ICEFEDES, if one takes letter pronunciation? The equivalent for Medieval Latin would have the "J" instead of the "I"?