The verb σῴζεσθε appears in the present passive indicative (second person plural form). So the idea is present tense. That is, Wu and Tan (2010) structure the sentence as follows; here the word σῴζεσθε modifies the words "the gospel" found in the previous verse. Please click to enlarge.
Thus the following translation would be accurate.
1 Cor 15:2 (Mounce Interlinear New Testament)
2 through which also you are being saved, if you hold firmly to the message I preached to you -- unless you have believed in vain.
Does this translation of σῴζεσθε therefore mean that salvation is a process, or a once-and-for-all state?
The Apostle James talks about the early and late rains of the Hebrew calendar, and the idea in the Hebrew Bible is that the early rains occur in the autumn during the Feast of Tabernacles (and Day of Atonement), but the latter rains occur in the spring during the Passover (and Feast of Unleavened Bread).
James 5:7-8 (NASB)
7 Therefore be patient, brethren, until the coming of the Lord. The farmer waits for the precious produce of the soil, being patient about it, until it gets the early and late rains. 8 You too be patient; strengthen your hearts, for the coming of the Lord is near.
In the Jewish calendar, during the Feast of Tabernacles (during the early rains of autumn), the Jews were in the wilderness wanderings. These wanderings were preparatory for entrance into the Promised Land. Most Jews in the wilderness wanderings however did not cross into the Promised Land. Joshua later crossed into the Promised Land after the Passover, which is the Feast of Unleavened Bread (during the latter rains of spring). The wide brushstrokes provide the idea that one is not "being saved" until the latter rains, when one enters the "rest" of the Promised Land.
Hebrews 4:1-3 (NASB)
1 Therefore, let us fear if, while a promise remains of entering His rest, any one of you may seem to have come short of it. 2 For indeed we have had good news preached to us, just as they also; but the word they heard did not profit them, because it was not united by faith in those who heard. 3 For we who have believed enter that rest, just as He has said,
“As I swore in My wrath,
They shall not enter My rest,”
although His works were finished from the foundation of the world.
The Hebrew calendar therefore helps to understand the difference between "being saved" as a process, and "being saved" as a final state. In the wilderness wanderings, when the early rains occurred (in the autumn of the desert), the seed took no root for lack of faith: here "salvation" was in process, but was in vain. During the latter rains (in the spring time), Joshua entered the Promised Land, where the battle with the giants had just begun: here "salvation" was a final state.
In the same passage of the same epistle, Paul draws this same distinction.
1 Cor 10:1-5 (NASB)
1 For I do not want you to be unaware, brethren, that our fathers were all under the cloud and all passed through the sea; 2 and all were baptized into Moses in the cloud and in the sea; 3 and all ate the same spiritual food; 4 and all drank the same spiritual drink, for they were drinking from a spiritual rock which followed them; and the rock was Christ. 5 Nevertheless, with most of them God was not well-pleased; for they were laid low in the wilderness.
Paul says that the Jews in the wilderness had the "sacrament" of baptism (were baptized in Moses), and had the "sacrament" of the bread and cup (spiritual food and spiritual drink), but were not "being saved" in the final sense, because their faith was in vain. In a few more chapters, the reader comes to Chapter 15, where Paul reiterates "being saved" in the final sense.
1 Cor 15:3-5 (NASB)
3 For I delivered to you as of first importance what I also received, that Christ died for our sins according to the Scriptures, 4 and that He was buried, and that He was raised on the third day according to the Scriptures, 5 and that He appeared to Cephas, then to the twelve.
These verses are the gospel message, which must be combined with faith. Mere knowledge, or mental assent, is no different than receiving rain (early rains) in the desert from heaven--the outlook is preparatory. However, when combined with faith, that same rain from heaven (latter rains) results not only in the Passover (Feast of Unleavened Bread) but also in the Feast of First fruits (Pentecost), which occurred 50 days after entering the Promised Land. In the Hebrew Bible, that 50th day marked the giving of the Law of Moses, but in the Christian New Testament, that same date was the giving of the Holy Spirit.
In conclusion, "being saved" is both a process ("early rains"), and a final state ("latter rains").
Wu, A., & Tan, R. (2010). Cascadia Syntax Graphs of the New Testament: SBL Edition. Bellingham, WA: Lexham Press.