HOW DO WE KNOW THAT THE OLD TESTAMENT PROMISES ARE VALID FOR THE GENTILES, TOO?

How do we know that the promises that God made to His chosen people, Israel, are valid for us, the Gentiles, too? Because in Jesus Christ, we become the children of promise, as Isaac was to Abraham. The apostle Paul explains in detail in Romans, chapter 4, that God considered Abraham's faith as his righteousness, before being circumcised. In Abraham, we the Gentiles, inherit all the promises made by God to His children. God had called Abraham "the father of many nations.” In verse 11 we read, "Then he received the sign of circumcision, a seal of the righteousness that he had by faith while uncircumcised, that he might be the father of all who believe, though they be not circumcised; that righteousness be imputed to them also” 

Romans 4:16
"Therefore it is of faith that it might be according to grace, so that the promise might be sure to all the seed, not only to those who are of the law, but also to those who are of the faith of Abraham, who is the father of us all"”

Those of us who were not born from Hebrew parents, are the branches grown from Isaac (Abraham's promised son), who by being "grafted" into Jesus Christ, become partakers of the "root and fatness" of the olive tree; If the root is holy and blessed, so are the branches. The "root" consists of Abraham, Isaac and Jacob, the patriarchs of Israel. God formed the Hebrews as a people, and are called "olive trees," we instead, come from nations that have been "wild olive trees." The Jews who did not keep the covenant with God, are cut branches. The warning written by Paul is very clear: we remain in the goodness of God, as long as we live in obedience to Him.

Romans11:16-17, 20-21
"For if the firstfruit is holy, the lump is also holy; and if the root is holy, so are the branches. And if some of the branches were broken off, and you, being a wild olive tree, were grafted in among them, and with them became a partaker of the root and fatness of the olive tree"

"Well said. Because of unbelief they were broken off, and you stand by faith. Do not be haughty, but fear. For if God did not spare the natural branches, He may not spare you either."

Acts 2:39
"For the promise is to you and to your children, and to all who are afar off, as many as the Lord our God will call.””

Paul writes to the Christians of Ephesus, who were not Jews, but Gentiles, showing them how through Jesus Christ they received the same rights as the people of Israel.

Ephesians 2:12-20
"that at that time you were without Christ, being aliens from the commonwealth of Israel and strangers from the covenants of promise, having no hope and without God in the world. But now in Christ Jesus you who once were far off have been brought near by the blood of Christ. For He Himself is our peace, who has made both one, and has broken down the middle wall of separation, having abolished in His flesh the enmity, that is, the law of commandments contained in ordinances, so as to create in Himself one new man from the two, thus making peace, and that He might reconcile them both to God in one body through the cross, thereby putting to death the enmity. And He came and preached peace to you who were afar off and to those who were near. For through Him we both have access by one Spirit to the Father. Now, therefore, you are no longer strangers and foreigners, but fellow citizens with the saints and members of the household of God, having been built on the foundation of the apostles and prophets, Jesus Christ Himself being the chief cornerstone"

The Corinthians were from Greece, so they too were also Gentiles.

2 Corinthians 1:20
"For all the promises of God in Him are Yes, and in Him Amen, to the glory of God through us.”

The Romans additionally were not Hebrews, but Paul also tells them that the promises are for them too, through Jesus Christ:

Romans 15:8-9
"Now I say that Jesus Christ has become a servant to the circumcision for the truth of God, to confirm the promises made to the fathers, and that the Gentiles might glorify God for His mercy, as it is written:“For this reason I will confess to You among the Gentiles, And sing to Your name.”"

Therefore the promises of the Old and New Testaments are valid for all of us that are in Christ Jesus. Let's take a look at some of them and ask the Holy Spirit to help us to believe and see how they apply to us each and all.

We need to strengthen our faith, believing and repeating the promises of Scripture that refer to the many difficult situations that we all face, until we are completely standing on the promises that the Lord will do the same for us. Only “faith” brings the fulfillment of God's promises.

"But without faith it is impossible to please Him, for he who comes to God must believe that He is, and that He is a rewarder of those who diligently seek Him.” (Hebrews 11:6)