On second thought, I will give an interpretation to the verse you herein posted. Do not want to be seen in the same light as those who ignore verses.
You gave:
“And you also were included in Christ when you heard the message of truth, the gospel of your salvation. When you believed, you were marked in him with a seal, the promised Holy Spirit…”
Ephesians 1:13
First of all this is a terrible translation. Don't know how you would get this from the Greek Text: “And you also were included in Christ".
This verse has to be taken in context, with the previous verses. It does not stand alone nor do any Bible verses. Paul had wrote, leading up to this verse, many great things.
Eph 1:4 even as he chose us in him before the foundation of the world, that we should be holy and without blemish before him in love:
Eph 1:5 having foreordained us unto adoption as sons through Jesus Christ unto himself, according to the good pleasure of his will,
Eph 1:6 to the praise of the glory of his grace, which he freely bestowed on us in the Beloved:
Eph 1:7 in whom we have our redemption through his blood, the forgiveness of our trespasses, according to the riches of his grace,
Eph 1:8 which he made to abound toward us in all wisdom and prudence,
Eph 1:9 making known unto us the mystery of his will, according to his good pleasure which he purposed in him
Eph 1:10 unto a dispensation of the fulness of the times, to sum up all things in Christ, the things in the heavens, and the things upon the earth; in him, I say,
Eph 1:11 in whom also we were made a heritage, having been foreordained according to the purpose of him who worketh all things after the counsel of his will;
So before we get to verses 13, we see that the following had already been stated. The writer magnifies God's purpose and action.
1) He chose us. We did not choose Him.
2) He foreordained us unto adoption.
3) According to His good pleasure of His will, not ours.
4) He made His Grace abound toward us.
5) According to His good pleasure which He purposed in Christ.
6) We were made a heritage.
7) Having been foreordained according to purpose and counsel of His will.
Strong verses that show all things done by God, for the ones Chosen.
Now verses 12-14:
Eph 1:12 to the end that we should be unto the praise of his glory, we who had before hoped in Christ:
This whole context, is about Paul explaining what has taken place to and for believers. The unbeliever is not even in view here. Nothing but "us" and "we" plural pronouns. Verse 12, starts with "to the end that we...." which explains that everything mentioned before, comes to a point for some purpose. That purpose is, that ".... we should be unto the praise of His Glory..." Therefore, the purpose of God's counsel, is that the chosen ones should be for the purpose of praising Christ and this is forever and ever. Thus, the chosen of verse 4, are the ones unto the praise of His glory.
Eph 1:13 in whom ye also, having heard the word of the truth, the gospel of your salvation,--in whom, having also believed, ye were sealed with the Holy Spirit of promise,
Eph 1:14 which is an earnest of our inheritance, unto the redemption of God's own possession, unto the praise of his glory.
"... having heard (past tense) the Word of Truth...." Having been given the ability to hear, you heard the Gospel. First however, one must have hearing ears!
Mat_11:15 He that hath ears to hear, let him hear.
Mat_13:9 He that hath ears, let him hear.
Mat_13:13 Therefore speak I to them in parables; because seeing they see not, and hearing they hear not, neither do they understand.
2Co_4:4 in whom the god of this world hath blinded the minds of the unbelieving, that the light of the gospel of the glory of Christ, who is the image of God, should not dawn upon them.
But when God opens the hearing and eyes of one then:
Mat 13:16 But blessed are your eyes, for they see; and your ears, for they hear.
"...in whom, having also believed..." (again, past tense), points to the time of ones first believing in Christ. Paul is just sating a fact here. You did indeed believe in Christ, His person and His work. He in no way is suggesting, that all of the things that came before, are do to believing. Rather, that the things mentioned before are the reasons for you belief. It is also the reason for your being sealed.
"...sealed with the Holy Spirit of Promise...." (again, past tense), we have been sealed. The believer is sealed for the purpose of ".... the earnest of our inheritance..." This seal takes place at the New Birth. The seal is not the New Birth, it is one of the many things given to the person, at the time of his/her regeneration (The New Birth). It is the King's Seal, that is administered over by the Holy Spirit, that legally binds us for the King of Kings. The King paid the redemptive ransom and His Servant, seals us to the promise, until His return.
2Cor 1:21 Now he that is establishing us with you in Christ, and anointed us, is God;
2Cor 1:22 who also sealed us, and gave us the earnest of the Spirit in our hearts.
While the Holy Spirit is the administrator of the seal.... verse 21, shows that all of the God-Head is involved.