Galatians 3:18 For if the inheritance be of the law, it is no more of promise: but God gave it to Abraham by promise.
In Galatians, Paul is showing us how our inheritance cannot be linked to what we do. God placed no law or conditions on us else grace is no more grace. In verse 18 above he shows how all that God promised Abraham came by a promise. What is meant by this promise is that God would do what He told Abraham He would do but at a future date and time. Galatians 3:8 says God preached the gospel to Abraham. God was preaching by promise. He was telling Abraham I will justify you in the Messiah. Not today but in the future.
Romans 4:17 (As it is written, I have made thee a father of many nations,) before him whom he believed, even God, who quickeneth the dead, and calleth those things which be not as though they were.
God calleth those things which be not as though they were. This is how it can be said in Romans 4:3 and it was counted unto him for righteousness. This was a promise. God promised Abraham that righteousness would come to him. Righteousness would be worked out by God on Abraham’s account. Form a time standpoint we know this did not happen during Abraham’s life.
Hebrews 11:13 These all died in faith, not having received the promises, but having seen them afar off, and were persuaded of them, and embraced them, and confessed that they were strangers and pilgrims on the earth.
Hebrews 11:39 And these all, having obtained a good report through faith, received not the promise:
Are there other instances in scripture where the Old Testament states something as completed though it would be fulfilled in Christ?
Psalms 2:7 I will declare the decree: the Lord hath said unto me, Thou art my Son; this day have I begotten thee.
Acts 13:33 God hath fulfilled the same unto us their children, in that he hath raised up Jesus again; as it is also written in the second psalm, Thou art my Son, this day have I begotten thee.
Psalms 22:1 My God, my God, why hast thou forsaken me? why art thou so far from helping me, and from the words of my roaring?
Matthew 27:46 And about the ninth hour Jesus cried with a loud voice, saying, Eli, Eli, lama sabachthani? that is to say, My God, my God, why hast thou forsaken me?
Both these verses in Psalms were fulfilled in Christ. Look in Isaiah.
Isaiah 53:12 Therefore will I divide him a portion with the great, and he shall divide the spoil with the strong; because he hath poured out his soul unto death: and he was numbered with the transgressors; and he bare the sin of many, and made intercession for the transgressors.
Luke 22:37 For I say unto you, that this that is written must yet be accomplished in me, And he was reckoned among the transgressors: for the things concerning me have an end.
Isaiah 61:1 The Spirit of the Lord God is upon me; because the Lord hath anointed me to preach good tidings unto the meek; he hath sent me to bind up the brokenhearted, to proclaim liberty to the captives, and the opening of the prison to them that are bound; 2 To proclaim the acceptable year of the Lord, and the day of vengeance of our God; to comfort all that mourn;
Luke 4:18 The Spirit of the Lord is upon me, because he hath anointed me to preach the gospel to the poor; he hath sent me to heal the brokenhearted, to preach deliverance to the captives, and recovering of sight to the blind, to set at liberty them that are bruised, 19 To preach the acceptable year of the Lord. 20 And he closed the book, and he gave it again to the minister, and sat down. And the eyes of all them that were in the synagogue were fastened on him. 21 And he began to say unto them, This day is this scripture fulfilled in your ears.
The fulfillment of these above verses awaited the incarnation of Christ. These could not be fulfilled until He was set forth. God could speak of these things as though they were done because they were of certainty to come about. God could speak of righteousness because He was certain of its coming. What He could not do though was to declare those in Christ righteous until that righteousness was manifest. Abraham believed God and His promises and awaited the day of Christ. By faith Abraham saw the coming of Christ.
John 8:56 Your father Abraham rejoiced to see my day: and he saw it, and was glad.
Romans 3:26 To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.
God could declare at this time; not before or after, but at this time. What time was this? It was that time when Christ was set forth as a propitiation. God would not be just to justify until that propitiation came satisfying the righteous and holy judgement of God.