“In this and the following verse the apostle shows how our calling is an argument that all things work together to advance our eternal happiness, by showing the several steps which the wisdom and goodness of God have settled, in order to complete our salvation.  In order to this he first gives us, in this verse, the foundation and finishing, or the beginning and end, of the scheme of our redemption:  For whom God did foreknow, he also did predestinate to be conformed to the image of his Son. To foreknow, here signifies to design before, or at the first forming of the scheme; to bestow the favor and privilege of being God’s people upon any set of men, Romans 11:2.  This is the foundation or first step of our salvation; namely, the purpose and grace of God, which was given us in Christ Jesus, before the world began, 2Timothy 1:9.  Then, he knew or favored us; for in this sense the word to know is taken in a great variety of places, both in the Old and New Testaments.  And as he knew the Gentiles then, when the scheme was laid, and before any part of it was executed, consequently, in reference to the execution of this scheme, he foreknew us.  This is the first step of our salvation, and the end or finishing of it is our conformity to the Son of God in eternal glory, Romans 8:17, which includes and supposes our moral conformity to him.  When God knew us, at the forming of the Gospel scheme; or, when he intended to bestow on us the privilege of being his people; he then destinated or designed us to be conformed to the image of his Son; and, as he destinated or determined us then to this very high honor and happiness, he pre-destinated, fore-ordained, or pre-determined us to it.  Thus we are to understand the foundation and finishing of the scheme of our salvation.  The foundation is the foreknowledge, or gracious purpose of God; the finishing is our being joint heirs with Christ.  Now, our calling or invitation stands in connection with both these.
1. It stands in connection with God’s foreknowledge; and so it is a true and valid calling: for we are called, invited, or chosen according to the foreknowledge of God the Father, who may bestow his blessings upon any people, as may seem good in his sight, 1Peter 1:2; consequently, we have a good title to the blessings of the Gospel to which we are called or invited.  And this was to be proved, that the Jew, to whom the apostle particularly wrote, might see that the Gentiles being now called into the Church of God was not an accidental thing, but a matter which God had determined when he conceived the Gospel scheme. Thus our calling is connected with God’s foreknowledge.
2. It stands also in connection with our being conformed to the image of his Son; for we are invited by the Gospel to the obtaining of the glory of our Lord Jesus Christ, 2Thessalonians 2:14.  And therefore, supposing, what the apostle supposes, that we love God, it is certain, from our being called, that we shall be glorified with the sons of God; and so our calling proves the point, that all things should work together for our good in our present state, because it proves that we are intended for eternal glory; as he shows in the next verse.  For we must understand his foreknowing, predestinating, calling, and justifying, in relation to his glorifying; and that none are finally glorified, but those who, according to his purpose, are conformed to the image of his Son.” Taylor. (Dr. Adam Clarke)

The foreknowledge of God here, does not intend his prescience of all things future; by which he foreknows and foretells things to come, and which distinguishes him from all other gods; and is so called, not with respect to himself, with whom all things are present, but with respect to us, and which is eternal, universal, certain, and infallible; for in this sense he foreknows all men, and if this was the meaning here, then all men would be predestinated, conformed to the image of Christ, called by grace, justified and glorified; whereas they are a special people, whom God has foreknown: nor is this foreknowledge to be understood of any provision or foresight of the good works, holiness, faith, and perseverance of men therein, upon which God predestinates them to happiness; since this would make something out of God, and not his good pleasure, the cause of predestination; which was done before, and without any consideration of good or evil, and is entirely owing to the free grace of God, and is the ground and foundation of good works, faith, holiness, and perseverance in them: but this regards the everlasting love of God to his own people, his delight in them, and approbation of them; in this sense he knew them, he foreknew them from everlasting, affectionately loved them, and took infinite delight and pleasure in them; and this is the foundation of their predestination and election, of their conformity to Christ, of their calling, justification, and glorification. (Dr. John Gill)