Everyone who believes that Jesus is the Christ has been born of God, and everyone who loves the Father loves whoever has been born of him. (1 John 5:1 ESV)
1 John 5:1 is a favourite “proof-text” by Reformed authors to prove their understanding of the ordo salutis, with regeneration preceding one being the recipient of (saving/true) faith.
Reformed apologists are correct, up to a point. The phrase "everyone who believes" is the Greek ὁ πιστεύων, which is a present active participle (lit. "the believing one"; cf. its usage in John 3:16) while the term "born" in the phrase "born of God" is γεγέννηται, which is the perfect passive of γενναω. Clearly, based on the theology of this verse, regeneration precedes the faith in view in 1 John 5:1, so faith is not the instrumental means of regeneration and one’s initial justification.
However, what many Reformed apologists tend to ignore is the question of what John viewed to be the instrumental means of this regeneration. In John 3:3-5, water baptism is clearly taught to be the instrumental means of one being "born again/from above." See my exegesis of this pericope here.
Furthermore, within the context of 1 John 5 itself, water baptism is the instrumental means of one being spiritually begotten/regenerated. In 1 John 5:6, 8, we read the following:
This is he that came by water and blood, even Jesus Christ, not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth . . . And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one.
These verses, especially when read in light of John 3:3-5 (as well as the myriad of other texts supporting the biblical teaching of baptism being salvific) supports the LDS view on soteriology and the ordo salutis, but not the Reformed understanding thereof.
It is only through the rebirth of water and of the spirit that one puts on Christ (per Gal 3:26-27) becomes a true believer. That is why John makes reference to Christ coming "by water and blood" (v.6) as water baptism is the instrumental means through which Christ's "blood" (atoning sacrifice) is applied to an individual; only by baptism and being the recipient of the benefits of Christ's atoning sacrifice does one become a son of God through adoption. V.8 strengthens this, where the Spirit, water, and blood are said to be "one"--in the act of baptism, the spirit of God sanctifies when one is baptised, while the "blood" of Christ cleanses the person from their past and then-present sins.
As with many claims by Reformed apologists, at best, they are advocating a half-truth. When one examines the context of 1 John 5, as well as the entirety of Johannine literature itself, John taught one was born again/from above through water baptism, resulting in one having the faith in view in 1 John 5:1.