Blessed be the God and Father of our Lord Jesus Christ! According to his great mercy, he has caused us to be born again to a living hope through the resurrection of Jesus Christ from the dead, to an inheritance that is imperishable, undefiled, and unfading, kept in heaven for you, who by God's power are being guarded through faith for a salvation ready to be revealed in the last time. - 1 Peter 1:3-5
Later in the chapter (starting at verse 13) Peter begins to explain that we are called to holy living and the reason he gives is that we have been born again:
He was foreknown before the foundation of the world but was made manifest in the last times for the sake of you who through him are believers in God, who raised him from the dead and gave him glory, so that your faith and hope are in God. Having purified your souls by your obedience to the truth for a sincere brotherly love, love one another earnestly from a pure heart, since you have been born again, not of perishable seed but of imperishable, through the living and abiding word of God - 1 Peter 1:20-23
This phrase "having been born again" (αναγεγεννημενοι) is:
a participle derived from the verb marked similar below. Its tense is perfect (which indicates a present-tense report of an action that has been completed but has effects in the now; like: "he has done"), its voice is passive (which indicates that the subject receives the action in stead of performs it), and its mood is participle (i.e. a verbal form that's used as an adjective or adverb). This form's case is nominative (which usually indicates subject), its number is plural, and its gender is masculine.
This is much like the statement of John 5:24:
Truly, truly, I say to you, whoever hears my word and believes him who sent me has eternal life. He does not come into judgment, but has passed from death to life.
Where "has" (εχει) is
Its tense is present (which indicates that the action is in the now), its voice is active (which indicates that the subject performs the action, in stead of receives it), and its mood is indicative (which describes a situation that actually is — as opposed to a situation that might be, is wished for, or is commanded to be).
And where "has passed" (μεταβεβηκεν) is
Its tense is perfect (which indicates a present-tense report of an action that has been completed but has effects in the now; like: "he has done"), its voice is active (which indicates that the subject performs the action, in stead of receives it), and its mood is indicative (which describes a situation that actually is — as opposed to a situation that might be, is wished for, or is commanded to be).
Just as Paul often refers to the previous state of "dead in trespasses and sins" as opposed to the present state of "alive in Christ", so the one who has believed in the name of the only begotten Son of God goes from being dead to being alive.
That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. Do not marvel that I said to you, ‘You must be born again.’ - John 3:6-7
That which is born of flesh (and flesh only) is dead (spiritually) and does not have spiritual life until "born again" by the grace of God through faith in Jesus Christ. This is an actual event that takes place immediately in the present upon repentance of sin and faith in the Son of God and has continuing effect unto eternity, hence it is said that the believer has eternal life and will never be condemned.
Faith in a misrepresented or misunderstood Christ, one who is less than the only begotten Son of God, does not effect the new birth and many who are called Christian have yet to be born again because they have not believed in the name of God's only Son.
Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name of the only Son of God. - John 3:18