I had a friend in college who believed OSAS. He had made an altar call and was baptised at 11 years of age. He didn’t drink, smoke or curse (ok, maybe a little cursing), but he was a womanizer. I asked him point blank what he thought would happen if he was engaged in adultery when the Lord came back to rapture His church. He looked me right in the eye and said, “She’ll be very surprised when she finds herself all alone.” I agree that OSAS is not scriptural, and that it is a lie right out of hell itself.
However, I do beleive in the security of the believer (not the same thing as OSAS). John 6:37 says, “All that the Father giveth to me shall come to me; and him that cometh to me, I will not cast out.” (DR). Notice, the Father gives people to Jesus, and He will not cast them out. Even when Peter confessed that Jesus was the Christ, Jesus told him that this was given to him by the Father. As for the question of Judas Iscariot, Acts 1:16 makes it clear that he was chosen to fulfill prophecy. I think the parable of the sower is helpful in understanding this. The seed is the Word. Some falls on hard ground and is eaten up by the birds (I think we all know people who have hard hearts towards God). Some falls among rocks, grows up quickly, but withers and dies when the sun comes up. These are people who “believe” (they make an altar call, get baptized and join a church), but when persecution comes along, they fall away (Paul mentions some like this by name in some of his epistles). Some seed falls among weeds and thorns. They grow, but they get choked by the weeds and thorns. Ananias and Sapphira are examples of this (see Acts 5). Finally, there is seed that falls on good ground and produces fruit. These are the true believers who the Father has given to the Son (read all of John 6).
As for why Paul gives warnings in his epistles, remember, he is writing to the churches, and there are people there whose hearts are “stoney ground” and “among weeds and thorns” alongside true believers. We do need to check ourselves, and Paul (and others) gives us a good measure to check ourselves against. Even John says, “They went out from us, but** they were not of us**. For if they had been of us, they would no doubt have remained with us; but that they may be manifest, that they are not all of us.” (1 John 2:19 DR emphasis mine). Saving faith is a continuous looking to Jesus, not a one time, emotion filled response to a sermon, shaking a preachers hand, baptism, etc…
And where is our “free will”? Isn’t God the potter? Aren’t we the clay? Does the clay tell the potter what it wants to be? Does the clay seek out the potter? Did God ask Pharoah if He could harden his heart? Did God ask Paul permission before striking him down on the road to Damascus? As Jesus says, “You have not chosen me: but I have chosen you; and have appointed you, that you should go, and should bring forth fruit; and your fruit should remain: that whatsoever you shall ask of the Father in my name, he may give it you.” (John 15:16 DR emphasis mine).
Hope this answers some questions.