"Have you suffered so much for nothing - if it really was for nothing?" NIV translation
Paul reminds them that they did not just have one experience of salvation through the Holy Spirit but have had many experiences of seeing the Spirit in action in their lives. It is easy for us as humans to discount a single event but when a bunch of events happen and they all point to the same thing then it is a lot harder to ignore. That is why Paul is reminding them of this point and he is confident that their experiences were not in vain. This combination of experiences over our lives as a Christian is what God uses to grow us closer in our walk with Him.
"Does God give you his Spirit and work miracles among you because you observe the law, or because you believe what you heard?" NIV translation
Paul is asking them this question to get them to look at the wonderful things that God is currently doing in their lives. He is reminding them of the fact that God did not just save them and forget about them but instead is at work in their daily lives. This points to the fact that, through the Holy Spirit, we have wonderful experiences with God each and every day. This daily walk is because of the relationship that has been repaired and not because of any works that we can do.
"Consider Abraham: 'He believed God, and it was credited to him as righteousness.' Understand, then, that those who believe are children of Abraham." NIV translation
Paul continues by reminding the Galatians that faith (belief in God) is the only way that anyone (including Abraham) has ever been made right with God. He also points out that those who believe are Abraham's children and not just the Jews. That is a reminder that our righteousness does not come from our family line but once again by faith.
"The Scripture foresaw that God would justify the Gentiles by faith, and announced the gospel in advance to Abraham: 'All nations will be blessed through you.' So those who have faith are blessed along with Abraham, the man of faith." NIV translation
In Genesis 22:17 & 18, God told Abraham that he would be blessed and have children from all nations. God was announcing to the world that salvation would be available to non-Jews as well as to the Jewish people. That is the blessing that is referred to in this passage and we see that the blessing (salvation) comes by faith for us just as it did for Abraham and our Jewish brothers. We are blessed through Abraham because he was obedient to God. There are those who believe that we (Gentiles) are kind of a plan B for God in that, when the Jews rejected Jesus, He, then, turned to us but this passage shows that we were a part of God's plan from the beginning. (God does not need a plan B).