If your spiritual salvation can be lost or even forfeited, the Scriptures lie, and are worthless.
Acts 16:30-31 it the only place in the Bible where the question is both asked and answered: What must I do to be saved?
The answer tells us: "Believe"... "believe" is punctiliar action. It can last a lifetime or it can last a moment. But, "Believe on the Lord Jesus and you will be saved." There are no maybe's in there. It's indicative. It's something that
will happen, no matter what, unless God is incapable of making happen, and he's fully capable.
There are no conditions. It doesn't say, "Believe and follow with good works, enduring until the end." It doesn't say, "Believe, and you may or may not be saved." It says, "Believe and you
will be saved."
However, this assurance is not a license to sin. You can commit every sin and the book, and you're still spiritually saved forever and ever. But, you are responsible for your works in the flesh, and at the Judgment Seat of Christ, you will be paid for the works you have done, whether good or bad.
Assurance is not a free pass to behave however you want.
Here's a study on salvation:
Salvation