In the conversation between Christ and Nicodemus, recorded in John’s Gospel, the Saviour showed that there are no spiritual activities without regeneration; in his first epistle, John labours the converse truth that there is no regeneration without spiritual activities. The fruits of regeneration are repentance, faith and good works. The regenerate believe rightly in Jesus Christ (1 John 5:1). They do righteousness (2:29). They do not live a life of sin (3:9; 5:18; the verbs ‘commits sin’, ‘cannot sin’, ‘does not sin’, express habitual actions, as the present tense regularly does in Greek, and not absolute sinlessness, as 1:8-10 makes clear). They experience faith’s victory over the world (5:4). They love their fellow-Christians (4:7). These are the marks by which the regenerate are known; for no man could do any of these things were he not born again. But we have no warrant for regarding anyone as regenerate without these marks. Any who lack them, whatever they may claim, are to be adjudged unregenerate children of the devil (3:6-10). Regeneration is known by its fruits.
Here, incidentally, is a sufficient answer to the question whether spiritual regeneration was a reality in Old Testament times. Fallen human nature was no less incompetent in spiritual things then than it is now. Had there been no regeneration in Old Testament times, there would have been no faith, and Hebrews 11 could never have been written.
From the chapter on regeneration in 18 Words: The Most Important Words You Will Ever Know by J. I. Packer.