We know Jesus gives eternal life because He declared Himself as the resurrection and the life, promising that those who believe in Him shall never die.
Jesus affirms His role as the source of eternal life in John 11 when He states, 'I am the resurrection and the life; he that believeth in me, though he were dead, yet shall he live' (John 11:25). This promise is confirmed in 1 John 5:11-12, where it is written that 'God hath given to us eternal life, and this life is in His Son.' Therefore, our assurance of eternal life rests not in our efforts but in believing in Christ, who Himself embodies life and resurrection.
Scripture References:
John 11:25, 1 John 5:11-12, John 10:28, Romans 5:12, John 6:40
Commentary