IT will help you to understand the first expression to which you refer if you notice the way in which the word “firstborn” is used in the Scriptures. It may of course be used in the strict literal sense of the eldest child of the family, but frequently it is used in a figurative sense, denoting one who holds the rights of the firstborn’s place, one who is heir. If you refer to Deuteronomy 21:1616Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn: (Deuteronomy 21:16) you will find in that one verse the word used both figuratively and literally.
A Scripture which also is very much to the point is Psalms 89:27,27Also I will make him my firstborn, higher than the kings of the earth. (Psalm 89:27) where God is represented as saying of David “I will make him My firstborn, higher than the kings of the earth.” David then was raised up of God and earthly sovereignty deputed to him as God’s firstborn or heir. In this he was a type of Christ. When we come to the New Testament we find hover that the position is vastly expanded. The heirship of the Lord Jesus not only extends over the kings of the earth but to the remotest bounds of creation. He is the Firstborn of all creation. Every right connected with all created things is vested in Him, And why? Because, “By Him were all things created,” as the next verse goes on to say. He holds the Creator’s rights in His own creation. How strong a proof is this of the absolute deity of the Lord Jesus.
Your second question is as to a Scripture which also most strongly asserts His deity and yet uses terms which most plainly declare the reality of His manhood. “Unto the Son He saith, Thy throne O GOD...” Yet before the quotation from Psalms 45 is finished we read the words to which you have called attention. He is God yet God is His God. How can this be explained? Only by the recognition of His true and complete manhood. Having become Man, He is so really Man that God is His God, and also He has others associated with Himself as His fellows or companions. To find out who these companions are we have to read, on into chapter 2 and there we find Him acknowledging as His brethren the “many sons” whom He is leading to glory. They are “all of one” with Himself. This would have been impossible indeed had He remained as He was before His incarnation. Only by becoming Man and accomplishing redemption was it possible for us, redeemed men, to be so wonderfully associated with Him.