It is true that you cannot lose something you haven't received yet.
The problem with people that believe in OSAS is that they believe they have already received the fulfillment of the promise.
Ephesians 1:11-14
11 In whom also we have obtained an inheritance, being predestinated according to the purpose of him who worketh all things after the counsel of his own will:
12 That we should be to the praise of his glory, who first trusted in Christ.
13 In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise,
14 Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory. |
We have the promise that we will receive an inheritance in the redemption.
Until that time, we have been given the Holy Spirit who is the earnest (something that gives pledge, promise, or indication of what is to follow) that shows God plans to fulfill His promise and give us that inheritance.
We shall be truly saved in the resurrection of the righteous.
Once we have been saved in the resurrection of the righteous, we will always be saved.