1 John 3:1-2
Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not.
Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is.
When a man or woman confesses with his or her mouth the Lord Jesus Christ and believes in his or her heart that God raised him from the dead [Romans 10:9-10], that person receives holy spirit within. He or she is born again. With God's holy spirit as seed within us, we are the sons and daughters of God.
1 Peter 1:22-23
Seeing ye have purified your souls in obeying the truth [Romans 10:9-10] though the Spirit unto unfeigned love of the brethren, see that ye love one another with a pure heart fervently:
Being born again, not of corruptible seed, but of incorruptible, by the word of God, which liveth and aideth for ever.
The first time you were born you were the son or daughter of your natural father because his seed is in you. When you obey the truth as presented in Romans 10:9-10, God places His incorruptible seed in you and you are born again. Born again this time with spiritual seed from God. This makes God your Father and you an heir of God.
The Spirit itself beareth witness with our spirit, that we are the children of God:
And if children, then heirs, heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together.
The word "heir" means to receive a lot or portion. As children of God we have an inheritance or portion set aside for us. This verse also says that we are joint-heirs with Christ. A joint-heir is one who shares fully all that the Father has to give. It is not a portion divided between two heirs. It is a full portion given jointly to all the children. It is like a joint bank account. Our name appears on the account along with Jesus Christ's name and all the assets of the account belong to all the heirs. What an inheritance!
To open their eyes, and to turn them from darkness to light, and from the power of Satan unto God, that they may receive forgiveness of sins, and inheritance among them which are sanctified by faith that is in me.
When the Lord Jesus Christ spoke to Paul on the road to Damascus he charged him with this ministry of turning others from Satan unto God that they may receive an inheritance by believing on the Lord Jesus Christ. Part of the ministry of Paul was to make known this inheritance to those who would believe. That is why we find a knowledge of our inheritance written in the epistles given to the Apostle Paul.
And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father.
Wherefore thou art no more a servant, but a son; and if a son, then an heir of God through Christ.
Abba is an endearing term like "Daddy." God is our Father. We are his sons and daughters. We have an inheritance as heirs of God through the Lord Jesus Christ.
In whom [Christ] 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,
Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.
After we believed we received holy spirit from God making us his children. This spirit is the earnest of our inheritance. Just as earnest money is put down to secure a loan, God has given us holy spirit as a token of the inheritance that we will receive. If holy spirit is the down payment, the token, the earnest; what must the inheritance be like!
1 Peter 1:3-4
Blessed be the God and Father of our Lord Jesus Christ, which according to his abundant mercy hath begotten us again unto a lively hope by the resurrection of Jesus Christ from the dead,
To an inheritance incorruptible, and undefiled, and that fadeth not away, reserved in heaven for you,
Our inheritance awaits us in heaven. It is incorruptible as is the seed He planted in us when we were born again. It is part of the lively hope that we hold as we look toward the return of His son Jesus Christ.
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,
God is also anxiously awaiting that day for He has an inheritance in us just as we have in Him. WE are the riches of the glory of His inheritance. What a day that will be!