Question: I have several questions. 1) Why did the first Christians use the Greek word ICHTHYS? 2) Who is the "NEW ADAM" and 3) do you know two saints of the 20th century?
Response: I'll try to give you something to work with here:
1) Although the origins are murky, I have compiled a sketch of what is known and what may be surmised at the following link: Ichthys Explanation. The word is an acronym for the Greek words meaning "Jesus Christ, Son of God, Savior". That is about all that can be said with authority.
2) Scripture does not use the terminology "new Adam". Jesus Christ, as the God-Man, the Son of Man, and the one who solves the sin problem which Adam brought into the world (Rom.5:12-19), is the last Adam (1Cor.15:45), for He makes possible eternal life for all of Adam's progeny through faith in Himself. After salvation, we are all new persons in Jesus Christ, the Last Adam (2Cor.5:17; Gal.6:15; Eph.2:15; 4:23-24; Col.3:10; cf. Rom.6:4; 12:2; 2Cor.4:16; Eph.2:10). Perhaps your source is conflating these two ideas.
3) All believers in Jesus Christ are "saints", which is a Latin derived word translating the Greek word hagios - meaning a person made holy and set apart by God or "sanctified" (Rom.1:7; 1Cor.1:2; 6:2; 16:15; 2Cor.8:4; Eph.1:1; Phil.1:1; Col.1:2; Heb.6:10; etc.). So all of us who have put our faith in Jesus Christ and have been sealed by His Spirit are "saints" by the biblical definition of the word (which is why Paul, for example, often addresses his letters "to all the saints" in the Greek [though English translations of this phrase vary widely]: Rom.1:7; 1Cor.1:2; 2Cor.1:1; Eph.1:1; Phil.1:1).
For more information on point 3, see:
Peter #8 on the doctrine of sanctification
Peter lesson #13: "Sanctification"
The Satanic Rebellion #5: on the three phases of sanctification
Hope this is helpful,
Yours in our Lord Jesus Christ,