If we take the Magesterial teaching, the meaning does not mean that we are 100 percent assured of our salvation.
This would be an incorrect interpretation of this verse. Either you are misreading the “magisterial” interpretation (I placed this in quotes because as far as I can tell there has never been an infallible declaration or interpretation of this verse), or the “magisterial” interpretation is incorrect. I will explain why in a moment.
Now to me there is a big difference between “being saved” and “are saved” If I am drowning “being saved” means I am being drag to shored (into the boat). Where as, “are saved” I am already on the shore(in the boat).
Your question isn’t making a mountain out of a molehill, it is a legitimate question regarding soteriology. The verb in the passage is in the present indicative middle voice or passive voice (Greek does not have a form that distinguishes the passive or middle voice - this is determined by context). What this means is that the word which the douay rheims version translates as “are being saved” and the NIV and other versions translates as “are saved” is in the present tense. However, there is no reference to time for whether the action is something that began in the past and is continuing into the future, or whether it is undefined. So for the moment, we will discuss the indicative. The indicative says something that is certain, this is in contrast to the subjunctive which expresses something that is conditional or speculative. In other words, Paul is saying that this is happening and it is certain that it is happening. The middle voice means that the subject is performing an action for the benefit of himself. The passive voice would mean that the subject is the recipient or beneficiary of the action, but is not the one performing it. The NIV or other versions may be relying on Paul’s statements in Romans and elsewhere that God is the one who justifies in order to bank on the passive mood. In this context, it would be appropriate to render the verse either way because in English we would use either phrasing to communicate the middle or passive voice. It is not a reflection on whether it is a past action that needs completion, or whether the act is speculative or certain.
Now, let’s look at the passage: Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand, by which also you are saved, if you hold fast the word which I preached to you, unless you believed in vain.
In other words, Paul is saying that he preached the word to the brethren at Corinth and that if they received this word, and stand in this word, they certainly are saved by it. He prefaces this though at the end by saying unless they believed in vain. In other words, if they didn’t receive his word in faith, then his word has no power to save. That being said, Paul again, demonstrates through this verbiage, that if they received the word by faith, then they are saved.