With Respect to Him Being the Only Son of God
The Lord Jesus Christ was called the only Son of God to distinguish Him from the rest of God's children who are called children through love, faith and adoption. He is the Only Son, who has the same Nature, Essence and Divinity with God. Christ was called 'the Only Son' in the following places:
"No one has seen
God at any time. The Only Begotten Son, who is in the bosom of the Father, He has declared Him" (Jn. 1:18). This means that Christ declared God and we knew God through His seen Incarnate Son, whereas the Father can not be seen in His Divinity.