Apostle Paul was once a Jew and a persecutor of the Christians, however when the gracious Lord revealed Himself to Paul did he realise that all this time he had been fighting against the same God whom he was trying to defend. The God of the Israelites is the God of the Christians.
The fact was not that Christians had found a new God rather that the Jews has forgotten their God and thus did not recognize Him when He Who led them from the captivity of Egypt and Who established them in the promised land came amongst them, they crucified Him.
"For I could wish that I myself were accursed from Christ for my brethren, my countrymen according to the flesh, who are Israelites, to whom pertain the adoption, the glory, the covenants, the giving of the law, the service of God, and the promises; " Romans 9:3-4
Paul, in his epistle to the Romans is so deeply moved with the salvation of the Jews that he is even ready to be accursed if due to that the Jews could be saved, for he recalls the former glory and the great calling with which the Jews were called, however now they have lost all their blessedness as they went away from God.
There is wonderful mystery with which the people of God always function and through which they are always successful and joyous and the mystery is :
"Christ in you, the hope of glory" Colossians 1:27 (NKJV)
A Christian is no one without Christ and thus anything and everything that one does without God is futility for Christ Himself has said:
"for without Me you can do nothing." John 15:5 (NKJV) and Christ also said...
"If you abide in Me, and My words abide in you, you will ask what you desire, and it shall be done for you. By this My Father is glorified, that you bear much fruit; so you will be My disciples."
John 15:7-8 (NKJV)
We need to abide in Him and He needs to abide in us for our blessedness.
The Jews were void of God and thus Paul laments and mourns for them, however are we filled with God, with His blessed Spirit, is Christ truly in us and thus our hope of glory?