Here are some thoughts. The "Earnest" is a pledge or guarantee of a future reward.
The earnest of spirit is an assurance to believers of the redemption yet to come, a future receiving of the inheritance, or as Alex says a pledge of future "born again".
A pledge of future blessedness to come. This blessedness is the new heaven and new earth promised to Abraham and his seed (the Christ and his chosen)
Could it be, that as a pledge/assurance that the "seed" would inherit the kingdom, the earnest of spirit was given. Could this pledge be the gifts of the spirit? Is receiving the earnest evidence of obeying the right message??
Act 5:32 And we are his witnesses of these things; and so is also the Holy Ghost, whom God hath given to them that obey him.
Gal 3:5 He therefore that ministereth to you the Spirit, and worketh miracles among you, doeth he it by the works of the law, or by the hearing of faith?
Heb 2:3 How shall we escape, if we neglect so great salvation; which at the first began to be spoken by the Lord, and was confirmed unto us by them that heard him;
Heb 2:4 God also bearing them witness, both with signs and wonders, and with divers miracles, and gifts of the Holy Ghost, according to his own will?
Mar 16:17 And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues;
Mar 16:18 They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover.
The earnest of spirit (gifts of spirit) are evidence of obeying the correct gospel. evidence of the promise to come??
2Co 1:22 Who hath also sealed us, and given the earnest of the Spirit in our hearts.
2Co 5:5 Now he that hath wrought us for the selfsame thing is God, who also hath given unto us the earnest of the Spirit.
Eph 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,
Eph 1:14 Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.
1Co 12:7 But, unto each one, is given the manifesting of the Spirit, with a view to that which is profitable;
From G5319; exhibition, that is, (figuratively) expression, (by extension) a bestowment: - manifestation.