The whole multitude of the disciples began to rejoice and praise God with a loud voice for all the mighty works that they had seen; saying, Blessed be the King that cometh in the name of the Lord: peace in heaven, and glory in the highest.  Luke 19: 37-38
When Jesus made His triumphal entry into Jerusalem, He was hailed as King by the crowds that had seen His miracles during His public ministry.  They acknowledged Him as the wonder-working Messiah, whom God had been promising to send to His people for many centuries.
In the Old Testament, God the Father is also referred to by this name.  But the Jewish people rejected God’s kingship in favor of rule by an earthly king in the time of the prophet Samuel (see 1 Samuel 8:4-9).
Saul, Israel’s first king, was followed by many other kings throughout history.  But by Jesus’ time, no king had ruled over the Jews for a period of about five hundred years.  They thought their long-promised Messiah would be a powerful king who would restore their nation to its glory days as a political power.  When the crowds greeted Jesus as King when He entered Jerusalem, they were thinking of Him in these terms.
Jesus faced this problem throughout His ministry.  After the miracle of the feeding of the five thousand, He realized that the crowds “would come and take him by force, to make him a king” (John 6:15).  He avoided them by slipping away to a secluded spot on a nearby mountain.
There was nothing Jesus could do to avoid the crowds on the day when He entered Jerusalem.  But He rode a donkey, a symbol of humility and peace, rather than a prancing white horse, the steed of choice for military heroes of the day (see Matthew 21:1-5).  This showed that He was not a political king but a spiritual King-one who had come into the world to conquer sin and death.