The instant of conversion is when we believe on God in the context of what He has done for us in Christ (Jn 3:36), but what is it exactly that we actually believe in or about God and/or Christ that saves us?
Abram was justified the instant he believed in Jehovah (Ge 15:6), yet he evidently had a deep relationship with God prior to this time and was following Him the best he knew how. (1-4) Abram had obediently left his home and family to follow Jehovah (He 11:8), built an altar to worship Him (12:8) and called on His name. (13:3-4) Yet Abram’s belief in God did not justify Him.
The faith which justified Abraham came afterward (Ro 4:19-22), and was thus more than believing in God’s existence, that God should be worshipped, trusted and followed at any cost. Such beliefs are evidently typical of those seeking salvation and involve prerequisites to saving faith, but do not fully comprise it. (He 11:6)
We might think believing on Christ is equivalent to accepting the fact that Christ is God’s Son and that He rose from the dead, yet we find Christ Himself telling us that many who call Him Lord, evidently believing such obvious basics about Him, will ultimately be cast away, eternally condemned. (Mt 7:21-23) Many who claim to believe the historical facts about Christ aren’t obeying Him, showing that they don’t love Him (Jn 14:23), and He’s telling us in no uncertain terms that these folk don’t belong to Him. (26-27)
God says Jesus Christ is made a propitiation for us through faith in His blood. (Ro 3:24-25) In other words, the belief that justifies is an explicit, unwavering trust in and dependence on the efficacy of the blood of Christ for one’s personal justification before God: it is the blood that makes atonement for our soul. (Le 17:11) It’s knowing we’re justified by what Christ has done in shedding His blood and dying for our sin (1Co 15:3), becoming our sin (2Co 5:21), washing us from our sins in His own blood (Re 1:5), and imputing perfect righteousness to us. (Ro 4:23-25)
This is not the same as believing Jesus Christ died to provide an offer of salvation to the whole world, such that anyone may be forgiven of their sins. While this is certainly true (Jn 3:16), this belief in itself does not save anyone because it is not personal; it’s not about one’s own sin being atoned and paid for. This belief opens the door to salvation, but believing it does not get us through the door because something that applies to everyone, but does not in itself save anyone, cannot be proper grounds for our justification. Believing it gives us no personal assurance of eternal life.
We are justified as we become fully persuaded that what God has promised us in Christ He is able also to perform (Ro 4:21-22): that the blood of Christ and His atoning work has satisfied God in our personal case and has eternally justified us. (Is 53:11) This is a supernatural work in which God assures us of eternal life in Christ (1Th 1:5), based entirely on the work Christ has done in dying for us personally on the Cross, paying our sin debt to God, and creates in us a new nature (2Co 5:15) that loves Him (1Co 8:3) and obeys Him. (1Jn 3:9-10)