Why is He called God’s only begotten Son, since we also are children of God?

Because Christ alone is the eternal, natural Son of God. We, however, are children of God by adoption, through grace, for Christ’s sake.

Proofs

• Because Christ alone is the eternal, natural Son of God. (John 1:1–3; John 1:14; John 1:18; John 3:16; Romans 8:32; Hebrews 1; 1 John 4:9)
• We, however, are children of God by adoption, through grace, for Christ’s sake. (John 1:12; Romans 8:14–17; Galatians 4:6; Ephesians 1:5–6)