49 I am come to send fire on the earth; and what will I, if it be already kindled? 50 But I have a baptism to be baptized with; and how am I straitened συνέχομαι till it be accomplished! τελεσθῇ
51 Suppose ye that I am come to give peace on earth? I tell you, Nay; but rather division: 52 For from henceforth there shall be five in one house divided, three against two, and two against three. 53 The father shall be divided against the son, and the son against the father; the mother against the daughter, and the daughter against the mother; the mother in law against her daughter in law, and the daughter in law against her mother in law.
-- Luke 12:49-53 (KJV)
The cross was the baptism that Jesus was waiting to be baptized with, and he was being "straightened" συνέχομαι (συνέχω - Strong's G4912 - synechō), constrained/afflicted/pressed on all sides, till it was accomplished τελεσθῇ (τελέω - Strong's G5055 - teleō) finished/completed/brought to an end.
Why did he feel so pressed?
Because, before that baptism, he was tasked with the recruitment and preparation of those who would publish his Gospel in all the world. And this task taxed him greatly.
Jesus' death on the cross -- the baptism he was to be baptized with -- the ordeal that was the culmination of such a taxing sojourn on earth, one that so pressed his body, mind and spirit -- was done. So, he cried out, "It is finished!"
The figure of baptism is clearly seen in his death: he was tried as a sinner, soiled with the accumulated sin of mankind; he was judged, and sentenced to the cross; he was plunged beneath the waters of death; and he raised himself up, victorious over it all, glorified, to sit at the right hand of the Father.
When Jesus said to the Pharisees, "ye shut up the kingdom of heaven against men: for ye neither go in yourselves, neither suffer ye them that are entering to go in.", he knew that task now complete, would open the doors wide to kingdom of God:
9 Surely the isles shall wait for me, and the ships of Tarshish first, to bring thy sons from far, their silver and their gold with them, unto the name of the LORD thy God, and to the Holy One of Israel, because he hath glorified thee.
10 And the sons of strangers shall build up thy walls, and their kings shall minister unto thee: for in my wrath I smote thee, but in my favour have I had mercy on thee. 11 Therefore thy gates shall be open continually; they shall not be shut day nor night; that men may bring unto thee the forces of the Gentiles, and that their kings may be brought.
-- Isaiah 60:9-11 (KJV)
It is finished! The work of opening the gates of the kingdom, so that every person who sees and/or hears of what Jesus endured to throw them open, and responds to it, can walk right through them into a realm where God dwells, and Jesus reigns as king, according to heavenly governance.