In Christ you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit. (Ephesians 1:13)

The word translated ‘sealed’ is the Greek verb σφραγίζω sphragizó describing the act of fixing a seal or stamp to something attesting to ownership.  This conveys the full authority, backing and security of the owner and at one time might also involve the use of tattoos; hence the derivation of this prophetic commandment to the world that John witnessed in his revelation:

‘Do not harm the earth or the sea or the trees, until we have sealed the servants of our God on their foreheads.’  And I heard the number of the sealed, 144,000, sealed from every tribe of the sons of Israel.  (Revelation 7:3-4)

So, in the head quotation, Paul is telling the faithful in Ephesus of their change of status when the Holy Spirit indwelt them.  This means they necessarily ‘holy’ or ‘set apart’; they now belong to God.

This is clearly important to the pagan Gentile converts, as Paul acknowledges:

… at one time you Gentiles in the flesh, called “the uncircumcision” by what is called the circumcision, which is made in the flesh by hands -remember that you were at that time separated from Christ, alienated from the commonwealth of Israel and strangers to the covenants of promise, having no hope and without God in the world.  (Ephesians 2:11-12)

The Jews were God’s chosen, claimed and taken out of Egypt, now God claims and sets apart others, not through the deliverance of Moses, but Jesus:

For through (Jesus) we both have access in one Spirit to the Father. So, then you are no longer strangers and aliens, but you are fellow citizens with the saints and members of the household of God… (ibid 2:18-19)

This was of course the greatest mystery concealed in the Hebrew texts, and Paul considered it was given to him (more so than the other apostles), in order he might…

… preach to the Gentiles the unsearchable riches of Christ, and to bring to light for everyone what is the plan of the mystery hidden for ages in God who created all things… (ibid 3:8b-9)

This in unity:

There is one body and one Spirit – just as you were called to the one hope that belongs to your call -one Lord, one faith, one baptism, one God and Father of all, who is over all and through all and in all. (Ibid 4:4-6)

And of course, in recognising that they were ‘sealed’ meant not only a change of status, but a change of ways.  Paul moves from what God has done for them, to what they most do for Him, for by carrying his imprimatur they necessarily act for Him, not against Him.  

you must no longer walk as the (other) Gentiles do, in the futility of their minds…putting off your old self, which belongs to your former manner of life and is corrupt through deceitful desires, and to be renewed in the spirit of your minds, and to put on the new self, created after the likeness of God in true righteousness and holiness.(Ibid 4:17&22-24)

Of course, Paul has to say this, because as once with the Israelites, the new people of God can labour under the same fatal misunderstanding and become presumptuous.  Paul witnesses (or at least anticipates) that on the former pagan Gentiles fully realising they are also chosen by God, them replicating the exact same mistakes as Israel.

It is human nature to fling God’s bounty back in his face, it started with Adam, and the Bible is fundamentally a list of God’s promises being dishonoured by humanity.

Again and again, Israel turned away from God, and there was no reason to think this a merely national and racial flaw, but simply human.

While 144,000 of Israel is a symbolic number, demonstrating God will deliver Israel, Paul also says that ‘not all who are descended from Israel belong to Israel’ (Romans 9:6).  Therefore, just as being an Israelite was not enough, for the Ephesians, simply joining a fellowship of faith was not sufficient.  Taking steps to walk with God was essential, leading to Paul exhortation to the Ephesian converts:

Therefore, be imitators of God, as beloved children. And walk in love, as Christ loved us and gave himself up for us, a fragrant offering and sacrifice to God. (Ibid 5:1-2)

Only then, can any derive assurance that they are ‘sealed’.   And Jesus is clear how to be imitators; by keeping the commandments of God, reiterated by Jesus, based on love of God and others.

‘This is my commandment, that you love one another as I have loved you.  Greater love has no one than this, that someone lay down his life for his friends.  You are my friends if you do what I command you. (John 15:12-14)