No one but Christ alone is known in the Scriptures as the “only begotten Son of God” (Jn 3:18; cf. Jn 1:18; 3:16; 1 Jn 4:9). This instructive description of the Lord has a twofold significance.
On the one hand, it distinguishes the Lord from believer. The Scripture calls believers “children of God” (Mt 5:9; Lk 20:36; Rom 9:8; etc.). But this is only because we have been adopted by God to be His children on account of our union with Christ, He “having predestinated us unto the adoption of children by Jesus Christ to himself, according to the good pleasure of his will” (Eph 1:5), and has given us the “Spirit of adoption, whereby we cry, Abba, Father” (Rom 8:15).
On the other hand, the phrase “only begotten Son of God” speaks of the eternal and natural relationship between the Father and the Son, or, in theological jargon, it speaks of the personal properties or the subsistence of the second person of the Trinity, namely, that He is eternally begotten of the Father. The significance of this designation of the Lord may be seen particularly in the words of the Apostle John in John 1:18—“No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him.” Notice how John highlights the fact that “the only begotten Son” has always been with His Father. In other words, He is the eternal Son of God. He did not become the Son of God, but has always been the Son of God, and remains the Son of God. This doctrine of eternal generation agrees well with the words of the Lord when He says: “For as the Father hath life in himself; so hath he given to the Son to have life in himself” (Jn 5:26).
When we consider the difference between the sonship of Christ with the Father and our own sonship with the Father, our hearts ought to be filled with wonder at the realisation that there is a vast difference between the two kinds of sonship, and yet, the Lord tells us: “As the Father hath loved me, so have I loved you” (Jn 15:9). No one can possibly imagine the love between the heavenly Father and His only begotten Son because they are of the same essence, and yet the Lord tells us that He loves us with the same love that His Father loves Him!
And not only so, but His love for us is unconditional, has no equal reciprocation from us, and yet is costly. “But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us,” says the Apostle Paul (Rom 5:8). Christ laid down His life for us in order to be a propitiation for our sin, i.e., in order to pay our debt of sin owed to God, by which we are estranged from God and are deserving of God’s infinite wrath. Thus Christ is our Redeemer. He shed His blood on the Cross of Calvary as our redemption price. In this way Christ delivers us from the wrath of the Father, and at the same time rescues us from the power of the devil who holds us captive to sin. This being so, we belong to Christ as slaves belong to their master. This is why we call Him Lord. But thanks be to God, He is a master we do not need to serve with servile fear, but with love and gratitude, for He Himself calls us His friends (Jn 15:15) and His brethren (Heb 2:12) on account of our adoption.