Sealed w Holy Spirt

Eph 1:13-14
13 In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise,
14 who is the guarantee of our inheritance until the redemption of the purchased possession, to the praise of His glory.

These two verses are encouraging to me. And I like to use them in the P of the "tuliP" argument.
(P is for
"Perseverance of the saints.") On this issue I agree with John Calvin.

The argument is that none who are truly saved can be condemned for their sins or finally fall away from the faith. The idea in sum is "once saved, always saved." And people who "fall away" from faith, were NOT ever truly saved in the first place.

Now this is a very controversal topic and again it is one of the non-essentials. What you think about this toppic in the end does not really matter, we are all going to live and get along together when we reach Heavan and have full understanding of the topic as God reveals these things to us.

I have recently read an interesting thought-provoking, but not convincing argument, "can the seal be broken?"
The more I read about their positions, the more I am confused by their understanding of Scripture...
Thet try to say that you can lose your salvation with one sin, but not all sins are of equal degree and then they try to examine scriptures and determine who is in Heaven and who is not due to their sins. King David is a good example. Based on his adultery is the "man after God's own heart" not in Heaven?

Some intersting reads, that is for sure.....