We are entering to-day upon the second part of the Christian church-year. The seasons and festivals of the church-year may be compared to a river that takes its rise, like the stream which washes the banks of our city, in some small and distant lake, and then ever continues to grow, widen, and deepen, until it becomes a majestic flow, and finally empties into the vast gulf of the ocean. We have seen in the past months the river of grace and salvation issuing as a tiny rivulet from under a humble manger on Bethlehem's plains, passing through the gorge of Nazareth, flowing along the banks of Jordan, sweeping past the cities of Galilee and Judea, lifting up its surging billows to the height of Calvary and Olivet, until it overflowed the world with its heavenly billows on the day of Pentecost. By that river it has been our good fortune to linger each Sunday, to dip up of its waters many a draught for our thirsty souls, and bathe in its currents for the washing away of our sins. To-day, however, we are called to ascend to its source, to leave Bethlehem, Nazareth, and Judea behind, to climb above Golgotha's Mount and Olivet's top, yes, to soar beyond the cloud which once received our ascended Lord out of sight, and to gaze upon a gulf, an ocean, which has no boundary and no shore. To speak in simple and unadorned speech: It is the subject of God Himself which we are invited to contemplate, the most overwhelming, mysterious, deepest of them all. "Who by searching," asks Job, "can find out God? Who can find out the Almighty to perfection?" And yet there are some things which we can and which we must know, for the subject of God is at the base of all things, of all religion. Without the right knowledge of God no man is a right man, and no one can rightly adjust himself to his place in this world or in the next. Let us, then, approach the great mystery of godliness, letting heavenly wisdom be our teacher.-- To-day's festival is called the Festival of the Trinity. What is the doctrine of the Trinity? For it certainly behooves every one to understand what is meant thereby, and this doctrine is held by all the Christian churches. Whosoever believes it, becomes a member of the Church. Whoever rejects it, ceases to belong to the Christian Church, and becomes a heretic. Scripture tells us on the one hand that God is one, that there are not three Gods, but one God; on the other hand, that the Father is God, that our blessed Lord Jesus, the Son, is God, that the Holy Ghost is God, each person being a perfect God, yet so joined, each to each, that they constitute one invisible God. We are taught that these three persons are uncreated, incomprehensible, eternal, almighty, equal in glory, majesty, and power. None is before, none after, none greater or less than another; they are coeternal and coequal.