From Herman Hoeksema’s The Triple Knowledge, vol. 2, pp. 80-84, on Heidelberg Catechism, Lord’s Day 19:

Thus the Catechism separates the ascension into heaven from the sitting at the right hand of God, which are combined into one article of faith in the Apostolic Confession.

The purpose of this is not to separate the two truths, or the two stages in the exaltation of our Lord, mentioned in this article, but rather to bring out that, although they are closely related, they are, nevertheless to be distinguished from each other. The ascension of Christ into heaven is the way unto his exaltation at the right hand of God; the latter is the end, the goal, of the former. The ascension was, no doubt, necessary unto the sitting at the right hand. In fact, we may safely say that, in the exaltation at the right hand of God, the entire glorification of the Saviour, beginning in the resurrection, finds its goal. And yet, the ascension and the sitting at the right hand of God, though so closely related, must be distinguished, and deserve a separate discussion. It is evident that the ascension is not the same as the exaltation at the right hand of God. It is possible to ascend to heaven, as in the case of believers, without being exalted to that highest position of power and glory that is indicated by the figure of sitting at the Right Hand.

Of this highest exaltation of Christ, Scripture speaks very frequently, and with great emphasis. It was clearly predicted in the old dispensation. Especially the psalms look forward to this universal glory of the universal king. In the Second Psalm we read of the eternal decree concerning this exaltation: "The Lord hath said unto me, Thou art my Son: this day have I begotten thee. Ask of me, and I shall give thee the heathen for thine inheritance, and the uttermost parts of the earth for thy possession. Thou shalt break them with a rod of iron; thou shalt dash them in pieces like a potter’s vessel" (vv. 7-9). If we read the Eighth Psalm in the light of Hebrews 2:6-10, it becomes plain that it speaks of this same dominion and power of our Lord: "For thou hast made him a little lower than the angels, and hast crowned him with glory and honour. Thou madest him to have dominion over the works of thy hands; thou hast put all things under his feet" (vv. 5-6). In Psalm 24 the people of God sing of the King of glory, who is Himself the Lord of hosts, but who ascends into the holy hill of Zion, and before whom the everlasting doors must be lifted up that He may come in. And in this general way, many more of the psalms, such as the forty-fifth, the seventy-second, the eighty-ninth, and others, speak of the glorification of the Servant of Jehovah, the theocratic king, the Christ of God. Moreover, the sitting at the right hand of God is literally mentioned in Ps. 110: "The lord said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool" (v. 1).

The Old Testament prophets, too, direct the hopeful eye of God’s people to this future exaltation and glory of the Messiah. For Jehovah’s servant "shall deal prudently; he shall be exalted and extolled, and be very high. As many were astonied at thee; his visage was so marred more than any man, and his form more than the sons of men: So shall he sprinkle many nations; the kings shall shut their mouths at him: for that which had not been told them shall they see; and that which they had not heard shall they consider" (Isa. 52:13-15). Daniel "saw in the night visions, and, behold, one like the Son of man came with the clouds of heaven, and came to the Ancient of days, and they brought him near before him. And there was given him dominion, and glory, and a kingdom, that all people, nations, and languages should serve him: his dominion is an everlasting dominion, which shall not pass away; and his kingdom that which shall not be destroyed" (Dan. 7:13-14). It is true that this passage looks even unto the second coming of our Lord, but so that His exaltation at the right hand of God is included in the scope of the vision. Zechariah is enjoined to take silver and gold of them which were of the captivity, and to "make crowns, and set them upon the head of Joshua, the son of Josedech, the high priest; And speak unto him, saying, Thus saith the Lord of hosts, saying, Behold the man whose name is the BRANCH; and he shall glow up out of his place, and he shall build the temple of the Lord; and he shall bear the glory, and shall sit and rule upon his throne; and he shall be a priest upon his throne: and the counsel of peace shall be between them both" (Zech. 6:12-13). That God’s priest, the Servant of Jehovah, shall be crowned with glory and honour, and be exalted to the kingdom, is clearly predicted in this passage.

It is hardly necessary to demonstrate that this truth is strongly emphasized in the new Testament. The Lord Himself, in the hour of His deepest humiliation, mentions His coming exaltation before the high priest: "Hereafter ye shall see the Son of man sitting on the right hand of power, and coming in the clouds of heaven" (Matt. 26:64). The disciples saw His power and coming, and were eyewitnesses of His majesty, when they were with Him in the holy mount (II Peter 1:16-19). And of that glory and majesty they speak very emphatically in their preaching of the gospel. In that wonderful sermon preached on the day of Pentecost, the apostle Peter proclaimed: "Therefore being by the right hand of God exalted, and having received of the Father the promise of the Holy Ghost, he hath shed forth this, which ye now see and hear. For David is not ascended into the heavens: but he saith himself, The Lord said unto my Lord, Sit thou on my right hand, Until I make thy foes thy footstool. Therefore let all the house of Israel know assuredly that God hath made this same Jesus, whom ye have crucified, both Lord and Christ" (Acts 2:33-36). And again, before the high priest and the counsel, he declares: "The God of our fathers raised up Jesus, whom ye slew and hanged on a tree. Him hath God exalted with his right hand to be a Prince and a Saviour, for to give repentance to Israel and forgiveness of sins." (Acts 5:30-31).

In the epistles, too, the truth of Christ’s exaltation at the right hand of God is repeatedly emphasized ...

We quoted all these passages, not so much to demonstrate that the confession concerning the sitting of Christ at the right hand of God is scriptural, as to bring out that the Bible lays great stress on this truth.

The sitting at the right hand is everywhere presented as a goal that has been reached.

It signifies that the first begotten of the dead has assumed his position as the firstborn of every creature.

Let us try, in the light of all that Scripture teaches us concerning it, to understand a little of the glorious mystery.