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Ephesians 1:13 In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise,
I think that this is a reference to the gifts of the spirit which were given to the Ephesian church by the laying on of Paul’s hands; something of which is recorded in Acts 19.
Acts 19:1 And it came to pass, that, while Apollos was at Corinth, Paul having passed through the upper coasts came to Ephesus: and finding certain disciples,2 He said unto them, Have ye received the Holy Ghost since ye believed? And they said unto him, We have not so much as heard whether there be any Holy Ghost.3 And he said unto them, Unto what then were ye baptized? And they said, Unto John’s baptism.4 Then said Paul, John verily baptized with the baptism of repentance, saying unto the people, that they should believe on him which should come after him, that is, on Christ Jesus.5 When they heard this, they were baptized in the name of the Lord Jesus.
6 And when Paul had laid his hands upon them, the Holy Ghost came on them; and they spake with tongues, and prophesied.
They were not sealed in the sense of a guarantee that they could not fall away because they had received gifts, but rather that the gifts were the setting of God’s seal to guarantee that the gospel preached was true.
All the churches established in the first century received a complement of holy spirit gifts for their establishment, edification and unification.
Ephesians 4:11 And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers;12 For the perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ:
13 Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ:14 That we henceforth be no more children, tossed to and fro, and carried about with every wind of doctrine, by the sleight of men, and cunning craftiness, whereby they lie in wait to deceive;15 But speaking the truth in love, may grow up into him in all things, which is the head, even Christ:
16 From whom the whole body fitly joined together and compacted by that which every joint supplieth, according to the effectual working in the measure of every part, maketh increase of the body unto the edifying of itself in love.
The gifts were given by the laying on of the apostle’s hands.
Acts 8:18 And when Simon saw that through laying on of the apostles’ hands the Holy Ghost was given, he offered them money,
Paul reminds the Corinthians that as a church they came behind in no gift.
1 Corinthians 1:7 So that ye come behind in no gift; waiting for the coming of our Lord Jesus Christ:
He speaks of his desire to visit the Roman church to impart some gift of the spirit upon them.
Romans 1:11 For I long to see you, that I may impart unto you some spiritual gift, to the end ye may be established;
Ephesians 1:14 Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.
The ‘earnest of our inheritance’ is a ‘pledge’ of that which is to come.It is described by the writer to the Hebrews as a taste of ‘the powers of the world to come’.
Hebrews 6:5 And have tasted the good word of God, and the powers of the world to come,
He goes on to indicate that if such fell away they would not again find repentance.
6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
Ephesians 1:15 Wherefore I also, after I heard of your faith in the Lord Jesus, and love unto all the saints,
Paul has heard of their progress in his absence and remembers them in his prayers that they might grow in their understanding of the truth.
Ephesians 1:16,17 I do not cease to give thanks for you, making mention of you in my prayers; That the God of our Lord Jesus Christ, the Father of glory, may give unto you the spirit of wisdom and revelation in the knowledge of him:
John 17:3 And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent.
Ephesians 1:18-20 The eyes of your understanding being enlightened; that ye may know what is the hope of his calling, and what the riches of the glory of his inheritance in the saints, and what is the exceeding greatness of his power to us–ward who believe, according to the working of his mighty power, which he wrought in Christ, when he raised him from the dead, and set him at his own right hand in the heavenly places,
Believing the gospel and being baptised into Christ is just a beginning.We begin as babes in Christ and must grow by applying ourselves to the word of God.
1 Peter 2:2 As newborn babes, desire the sincere milk of the word, that ye may grow thereby:
2 Peter 3:18 But grow in grace, and in the knowledge of our Lord and Saviour Jesus Christ. To him be glory both now and for ever. Amen.
Hebrews 5:12 For when for the time ye ought to be teachers, ye have need that one teach you again which be the first principles of the oracles of God; and are become such as have need of milk, and not of strong meat.13 For every one that useth milk is unskilful in the word of righteousness: for he is a babe.14 But strong meat belongeth to them that are of full age, even those who by reason of use have their senses exercised to discern both good and evil.
There is progress to be made in wisdom and understanding and this is what Paul is praying for for the Ephesians.
2 Peter 1:5 And beside this, giving all diligence, add to your faith virtue; and to virtue knowledge;6 And to knowledge temperance; and to temperance patience; and to patience godliness;7 And to godliness brotherly kindness; and to brotherly kindness charity.8 For if these things be in you, and abound, they make you that ye shall neither be barren nor unfruitful in the knowledge of our Lord Jesus Christ.9 But he that lacketh these things is blind, and cannot see afar off, and hath forgotten that he was purged from his old sins.10 Wherefore the rather, brethren, give diligence to make your calling and election sure: for if ye do these things, ye shall never fall:11 For so an entrance shall be ministered unto you abundantly into the everlasting kingdom of our Lord and Saviour Jesus Christ.
I hope this helps.