"When the Lord says to His people and children that He is the Lord their God, He thereby wants to assure them:
1. That He has entered into an eternal and immutable covenant with them in which He, in spite of their great misery through sin, embraces them as His own beloved children, being moved with pity and compassion regarding their most wretched condition. In so doing, He most securely binds Himself to them in order to be their merciful God and Father, and He engages all of His omnipotence to secure their eternal salvation and redemption, eternally bestowing upon them all good things. Consider what the Lord Himself is saying: "Moreover I will make a covenant of peace with them, and multiply them, and will set my sanctuary in the midst of them for evermore. My tabernacle also shall be with them: yea, I will be their God, and they shall be my people" (Ezek. 37:26-27).
2. Of their acquittal and the gracious pardon of all their sins for the sake of the merits of His Son, Jesus Christ, and that He is willing to engage all of His power, wisdom, grace, and mercy to secure the atonement of all their sins and iniquities by fully blotting them out, cleansing and purifying their hearts of them, and granting them a perfect and eternal righteousness. Yes, He assures them that He Himself will be their righteousness, so that they neither need to fear or doubt God's grace, no matter how great, grievous, and manifold their sins may be; that is, if they are but truly sorrowful regarding them and they are desirous to have them forgiven out of grace and for Christ's sake. Again, God Himself declares, "But this shall be the covenant that I will make with the house of Israel; after those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people. . .for I will forgive their iniquity, and I will remember their sin no more" (Jer. 31:33-34).
3. Of the great blessing of their sanctification. As the Lord Himself is infinitely holy in His nature, it necessarily follows that this God could not be our God without also perfectly sanctifying us and impressing deeply and fully upon our hearts His divine image. Thereby we become a holy people in conformity to that beautiful image of our God, whereby, from day to day, we are renewed and transformed until the full perfection of our nature will have been achieved.
Hereby the Lord is saying: "Fear not, My people, for though you find yourselves to be so very wretched, impotent, perverse, and sinful, and you are continually complaining and grieving that you are unable to love, serve, and fear Me with your whole heart and soul, I nevertheless assure you that I am the Lord thy God. I will engage all My power and all-sufficiency to make you completely holy in both soul and body, and to deliver you from all your sin. Be fully assured of this and entrust yourself entirely to Me. I have already begun My work in you, and I shall not forsake the work of My hands. Rather, I shall gradually proceed with it, and there will come a time when I shall completely perfect it. I, the Lord, am holy, and it is therefore My will that you, My people, shall ultimately be perfectly holy, and that to all eternity." It is therefore written, "that thou mayest be an holy people unto the LORD thy God, as he hath spoken" (Deut. 26:19), and "the heathen shall know that I the LORD do sanctify Israel" (Ezek. 37:28a).
4. That He will always inwardly illuminate them in their hearts by His Word and Spirit. He thereby teaches and instructs them so that they get to know and continue to know Him as their God, and, as the only wise God. He cures the spiritual blindness of their hearts and grants them His Spirit as the Spirit of wisdom and understanding. We may indeed mourn greatly because of our blindness, and we should be greatly troubled by the fact that we know so little of God and of heavenly things. However, since the Lord is our God, we have the most excellent teacher, for He has given us His only begotten Son to be our wisdom. We must therefore be of good courage, for we have but to wait upon and trust in the Lord, for He Himself is our light. He is the sun that illuminates our dark hearts, and He gradually causes all haze and cloudiness to evaporate. That sun will once break through fully, and He will bring forth our light as the noonday." pages 184-187 'The Christian's Only Comfort In Life And Death: An Exposition of the Heidelberg Catechism' Volume 2 by Theodorus VanderGroe