Summary: What do we know concerning the return of Christ the Lord? John points Christians to that day as a means of encouraging holy living.
“See what kind of love the Father has given to us, that we should be called children of God; and so we are. The reason why the world does not know us is that it did not know him. Beloved, we are God’s children now, and what we will be has not yet appeared; but we know that when he appears we shall be like him, because we shall see him as he is. And everyone who thus hopes in him purifies himself as he is pure.” 
There are truths concerning the return of our Lord that have never been revealed; we must wait until they are fulfilled to know the meaning of some things. As one example, recall that Jesus taught His disciples, “In those days, after that tribulation [frightful judgements on the whole earth], the sun will be darkened, and the moon will not give its light, and the stars will be falling from heaven, and the powers in the heavens will be shaken. And then they will see the Son of Man coming in clouds with great power and glory. And then he will send out the angels and gather his elect from the four winds, from the ends of the earth to the ends of heaven.
“From the fig tree learn its lesson: as soon as its branch becomes tender and puts out its leaves, you know that summer is near. So also, when you see these things taking place, you know that he is near, at the very gates. Truly, I say to you, this generation will not pass away until all these things take place. Heaven and earth will pass away, but my words will not pass away.
“But concerning that day or that hour, no one knows, not even the angels in heaven, nor the Son, but only the Father. Be on guard, keep awake. For you do not know when the time will come. It is like a man going on a journey, when he leaves home and puts his servants in charge, each with his work, and commands the doorkeeper to stay awake. Therefore, stay awake—for you do not know when the master of the house will come, in the evening, or at midnight, or when the rooster crows, or in the morning—lest he come suddenly and find you asleep. And what I say to you I say to all: Stay awake” [MARK 13:24-37].
We do not know precisely how we will be changed, though we are certain that we who have believed, we who are twice-born and who have the Spirit, will be changed. We are taught, “Our citizenship is in heaven—and we also await a savior from there, the Lord Jesus Christ, who will transform these humble bodies of ours into the likeness of his glorious body by means of that power by which he is able to subject all things to himself” [PHILIPPIANS 3:20, 21 NET BIBLE].
The Apostle Paul has written elsewhere, “Listen, I am telling you a mystery: We will not all fall asleep, but we will all be changed, in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised incorruptible, and we will be changed” [1 CORINTHIANS 15:51, 52 CHRISTIAN STANDARD BIBLE].
It is this truth, this divine promise of transformation into Christ’s likeness, that becomes the focus of our study this day. God has graciously revealed to His servants what they shall be at the return of the Master. The writings of the Apostle of Love will occupy our attention for these moments as we prepare ourselves for honourable service until that glorious day when the Master at last returns to take His redeemed people to Himself. Serving Him, we will prepare ourselves for the transformation that is coming, fulfilling the service to which He has assigned us.
WHAT WE ONCE WERE — Any study that seeks to understand what God has planned for His own people must begin with what we were when He called us. Our first parents were created to walk with God, living forever in sweet communion with Him. We are not told how long Adam and Eve lived in the Garden that God had created; we are not told how much time passed between the creation and the dark day when sin was introduced into the world. Nevertheless, sin did enter the world. We are taught, “Sin came into the world through one man, and death through sin, and so death spread to all men because all sinned” [ROMANS 5:12]. The passage refers us back to Adam and his rebellion against the Holy One in the Garden of Eden.
Our first parents sinned—our first mother, Eve, was deceived, but our first father, Adam, chose to rebel against the Lord God, his Creator. As resulting of their sinful rebellion, the whole of creation was plunged into ruin. This is the meaning of the words recorded in the Letter to Roman Saints: “We know that the whole creation has been groaning together in the pains of childbirth until now. And not only the creation, but we ourselves, who have the firstfruits of the Spirit, groan inwardly as we wait eagerly for adoption as sons, the redemption of our bodies” [ROMANS 8:22, 23]. The consequences of their sin included death for all mankind.