So likewise ye, when ye see these things come to pass, know ye that the kingdom of God is nigh at hand.

Wesley's Notes

21:31 The kingdom of God is nigh - The destruction of the Jewish city, temple, and religion, to make way for the advancement of my kingdom.

Matthew Henry's Concise Commentary

21:29-38 Christ tells his disciples to observe the signs of the times, which they might judge by. He charges them to look upon the ruin of the Jewish nation as near. Yet this race and family of Abraham shall not be rooted out; it shall survive as a nation, and be found as prophesied, when the Son of man shall be revealed. He cautions them against being secure and sensual. This command is given to all Christ's disciples, Take heed to yourselves, that ye be not overpowered by temptations, nor betrayed by your own corruptions. We cannot be safe, if we are carnally secure. Our danger is, lest the day of death and of judgment should come upon us when we are not prepared. Lest, when we are called to meet our Lord, that be the furthest from our thoughts, which ought to be nearest our hearts. For so it will come upon the most of men, who dwell upon the earth, and mind earthly things only, and have no converse with heaven. It will be a terror and a destruction to them. Here see what should be our aim, that we may be accounted worthy to escape all those things; that when the judgements of God are abroad, we may not be in the common calamity, or it may not be that to us which it is to others. Do you ask how you may be found worthy to stand before Christ at that day? Those who never yet sought Christ, let them now go unto him; those who never yet were humbled for their sins, let them now begin; those who have already begun, let them go forward and be kept humbled. Watch therefore, and pray always. Watch against sin; watch in every duty, and make the most of every opportunity to do good. Pray always: those shall be accounted worthy to live a life of praise in the other world, who live a life of prayer in this world. May we begin, employ, and conclude each day attending to Christ's word, obeying his precepts, and following his example, that whenever he comes we may be found watching.