"Then said Jesus to those Jews which believed on him, If ye continue in my word, then are ye my disciples indeed; And ye shall know the truth, and the truth shall make you free." (John 8:31-32)
One may come to a mental acceptance of certain facts about God and deceive himself into thinking he believes on Christ, but the genuineness of that faith needs to be tested before it is judged as real. "Thou believest that there is one God; thou doest well: the devils also believe, and tremble. But wilt thou know, O vain man, that faith without works is dead?" (James 2:19-20). Works, of course, do not save one's soul, but the faith that does save is a faith that produces good works.
An earlier group of listeners had also "believed in his name, when they saw the miracles which he did. But Jesus did not commit himself unto them |literally, 'did not believe on them'|, because he knew all men" (John 2:23-24). It is essential that we believe on Him, but even more essential that He believe on us!
The test is simple! If we continue in His Word, then we are real disciples. Our dedication to Christ is measured by our obedience to His Word. If we question His Word, or resent His Word, or try to avoid obeying His Word, by "interpreting" some preferred meaning of our own into His Word, then we urgently need to examine the genuineness of our belief in Christ.
If we are His disciples and do continue in His Word, then He makes us a wonderful promise: "Ye shall know the truth." Literally, this is "come to know the truth." As true disciples (that is, "learners"), we have His assurance that our continuing studies in His Word will yield an ever-increasing comprehension of God's created and revealed "truth," with ever-growing freedom to understand and obey the perfect will of God. HMM