Wondering,
Of course it is not necessary for Christians to know Greek. But I'm sure glad translators of the NT knew their Greek grammar in detail. I have been blessed beyond measure to know the grammar and nuances of Greek.
For you to say, 'OSAS does NOT exist in the N.T.', you have to be able to interpret Scripture. Hermeneutics is sometimes a daunting task. I happen to agree with you with this theology. However, Perseverance of the Saints is taught in Scripture.
Jesus spoke of the signs of the end of the age:
9 “Then you will be arrested, persecuted, and killed. You will be hated all over the world because you are my followers. 10 And many will turn away from me and betray and hate each other. 11 And many false prophets will appear and will deceive many people. 12 Sin will be rampant everywhere, and the love of many will grow cold. 13 But the one who endures to the end will be saved. 14 And the Good News about the Kingdom will be preached throughout the whole world, so that all nations will hear it; and then the end will come (Matt 24:9-14 NLT).
Perseverance to the end is a sign of true Christian belief and practice and not a person who once said, 'I asked Jesus into my heart', and then leaves Jesus out of the mainstream of his/her life.
Oz