Having stated in question and answer 119 that the Lord's Prayer is the model for prayer Christ gave to His people, the Heidelberg Catechism begins to exposit the Lord's Prayer in question and answer 120. The prayer begins by addressing God as "Father" (Matt. 6:9; Luke 11:2), so it is fitting that the catechism explores the reasons why Jesus tells us to speak to our Creator in this way.
As we consider what it means to call God our Father, we need first to address the serious misunderstanding of the fatherhood of God that is prevalent in our culture. The average person on the street, if he believes in a deity at all, is likely to hold the opinion that "we are all God's children." We invariably see the argument that "we are all God's children, and He loves us unconditionally" being used to allow impenitent adulterers, homosexuals, and "transgendered people" in the pulpit and at the Lord's Table in mainline Protestantism. Basically, most Westerners believe that all people are God's children simply because they are human beings.
This is not the biblical view of God's fatherhood. Certainly, the Bible refers to all human beings as God's "offspring" (Acts 17:28), but in so doing it speaks only of our Creator's role in bringing us into existence. There is a Creator-creature relationship between the Lord and all of humanity. This is far different, however, from the father-son relationship that the Bible has in mind when it refers to the Lord as our Father—a relationship in which we have personal, loving communion with our God and enjoy access to His presence (Rom. 5:1–2; Eph. 3:11–12). We are not born into this relationship simply by emerging from our mothers' wombs; rather, God must adopt us. This relationship is ours only when we are united to Christ by faith and receive "the right to become children of God" (John 1:12). We must be united to the only One who is the Son of God by nature, the Lord Jesus Christ. If we do not trust in the Savior, our relationship to God is merely that of a creature to its Creator, and we are under the Lord's holy wrath (Rom. 1:18–32).
Paul, in today's passage, tells us that we have been adopted as God's sons and daughters because of the work of Christ (Gal. 4:1–7). Because we are adopted, we have the assurance that we are His and that He will never disown us.
In the first-century Mediterranean world, adoption meant that a father could not disown his son or treat him any less than his natural-born son. By analogy, this great truth applies to God and His children—all those who trust in Christ alone are the beneficiaries of an abundant inheritance that cannot be lost (Deut. 21:15–17; Gal. 3:29). It is comforting indeed to know that we are God's children in Christ and that He will never reject those who hope in Him.