“Now after that John was put in prison, Jesus came into Galilee, preaching the gospel of the kingdom of God, And saying, The time is fulfilled, and the kingdom of God is at hand: repent ye, and believe the gospel.” - Mark 1:14, 15.
God Almighty had spoken in prophecy about Jesus Christ as the One through whom the door of faith would be opened to the gentiles, (Isaiah 49:6-8) Thus, when Christ started his Ministry his emphasis was on the Kingdom of God which could be attained by those who agree to serve God through His only begotten son.
Some Jews did not listen to Christ at that time. They called him names, made jest of him and looked for opportunities to fault him and even kill him. Things got to a point that, he cursed Jerusalem, saying: “O Jerusalem, Jerusalem, thou that killest the prophets, and stonest them which are sent unto thee...Behold, your house is left unto you desolate. For I say unto you, Ye shall not see me henceforth, till ye shall say, Blessed is he that cometh in the name of the Lord.” (Matthew 23:37-39) This prophecy fulfilled in AD 70 when Titus the Roman general destroyed Jerusalem, killed many of the people and sent the remaining ones into exile.
This should teach each person to repent and serve God diligently now because it is a fearful thing to fall into the hands of the living God. (Hebrews 10:31) St. Paul states, “For if God spared not the natural branches, take heed lest he also spare not thee. Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off.” - Romans 11:21, 22.