1 John 5:10
He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son. He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son.
Whoever believes on the Son of God has the witness in himself, while whoever does not believe the record God gave of His Son makes Him a liar. God's record is that He gives eternal life and that this life is in His Son, so that whoever has the Son has life, and the verse belongs to an assurance that believers may know they have eternal life and to an encouragement to trust in the Son. The inward witness belongs to those who see Christ as the truth and receive His testimony, in contrast to the world which cannot receive the Spirit of truth because it has not studied saving truth and does not know Christ.
The inward witness is conviction that God's word is true and that Jesus Christ is the divine Son of God. It is confirmed in the converted person's own experience of help, satisfaction, assurance, and deliverance found in Jesus after passing from death to life, by which they know the Bible as God's voice and the revelation of Christ and know Jesus as a divine Saviour and not a fable. Accepting the evidence of God's word brings faith, hope, and assurance that act as a barrier and guard against infidelity. The inner witness grows through overcoming obstacles and opposition in Christ's name and faithfully carrying responsibilities, bringing personal knowledge of Christ's faithfulness and power rather than reliance on the experience of others.