Salvation is a free gift: we may be justified freely, forgiven of all our sin, by God’s grace through the redemptive work of Christ (Ro 3:24). Christ becomes our propitiation (that which reconciles us to God) through faith in Christ’s blood. (25) There are no strings attached; we don’t have to earn salvation; it’s a totally free gift, and it must be free: we could never earn it.
However, this gift of eternal life doesn’t stand all by itself, all on its own: the gift is part of, or comes along with, a divine Person: Jesus Christ. We receive the gift of salvation, of eternal life, by receiving Christ (Jn 1:12), by believing on Him. (Jn 3:36) We cannot take the gift of salvation without also taking Christ, and vice versa — they go together. (1Jn 5:11-12)
Now, this believing in or on Christ isn’t simply believing He existed, or that He is alive even now; we can believe someone exists without believing in them. It isn’t even about whether we like them or are friends with them. We believe in someone when we find them trustworthy, dependable, faithful, capable, reliable; we take them at their word and act on what they say without question, as if it’s absolutely true. (Jn 3:32-33)
We also treat them consistently with who they claim to be, giving them the respect and honor they deserve. (Jn 5:23) We can’t rightly say we believe in someone, or that we receive them, unless we’re treating them this way; both taking them at their word, acting on it as true, trusting confidently in them, and respecting and honoring them as we should.
So, believing on Christ is no ordinary thing, no natural thing (Mt 19:25-26), and this is rooted in Who Christ Himself claims to be: Christ claims to be Lord of all, to have the right to inspect and order all our actions, even our inmost thoughts and affections; He is to be obeyed on every level, all the time. (Lk 6:46) A person who isn’t willing to submit to Christ as Lord in this way doesn’t believe in Him or receive Him (Jn 14:23-24); rather, they are rebelling against Him, at enmity with Him. (1Jn 2:3-5) All of us who belong to Him, who are justified by Him, do submit to Him in this way. (Ep 5:24) It doesn’t mean we’re perfect yet, but we want to be and are doing our best to obey Him. (1Jn 3:3)
And believing in Christ, receiving Him, is also more than obeying Him, more than following Him; it includes relying on Him, trusting Him in what He said He came to do: to seek and to save that which was lost. (Lk 19:10) This involves understanding what Christ has done to save us by dying for our sins (1Pe 2:24), confidently trusting He has taken care of our sin debt, knowing God Himself is satisfied with what Christ has done for us. (Is 53:11)
Both of these aspects of believing on Christ require a supernatural work of grace in our hearts, which is also a free gift (Ep 2:8), where God quickens us, delivers us from the power and dominion of sin (Mt 1:21), begins transforming our nature from one of disbelief, resentment and rebellion into one of submission, trust and love. (Tit 2:14)
And when God does this work in our hearts and we believe on Christ in this way, righteousness is imputed to us as a free gift (Ro 4:23-24), an eternal gift rooted and grounded in a new, supernatural, believing nature within us (2Co 5:17), which is itself the work of God. (Jn 6:29)