77 But because this belongs to few, He now begins to speak of searching for and possessing wisdom, which is a tree of life; and certainly, in searching for and possessing, i.e. contemplating this wisdom, such an eye is led through all that precedes to a point where there may now be seen the narrow way and the strait gate. When, therefore, He says in continuation, Enter ye in at the strait gate: for wide is the gate, and broad is the way, that leads to destruction, and many there be which go in thereat: because strait is the gate, and narrow is the way, which leads unto life, and few there be that find it; He does not say so for this reason, that the Lord's yoke is rough, or His burden heavy; but because few are willing to bring their labours to an end, giving too little credit to Him who cries, “Come unto me, all you that labour, and I will give you rest.
Take my yoke upon you, and learn of me; for I am meek and lowly in heart: for my yoke is easy, and my burden is light” (hence, moreover, the sermon before us took as its starting-point the lowly and meek in heart): and this easy yoke and light burden which many spurn, few submit to; and on that account the way becomes narrow which leads unto life, and the gate strait by which it is entered.
Source: Our Lord's Sermon on the Mount (New Advent)