John Chrysostom was not a Pelagian and the claim that "early synergist fathers implicitly believed same that man can be saved without grace" is patently absurd.
In his initial defense against Pelagius, Augustine took an extreme view that man is saved entirely by grace without any involvement of man's free will whatsoever. In "On Rebuke and Grace" he wrote:
Will you dare to say that even when Christ prayed that Peter's faith might not fail, it would still have failed if Peter willed it to fail? As if Peter could in any measure will otherwise than Christ had wished for him to will?1
Several Fathers came out against Augustine's extreme views - which Augustine himself later came to renounce later in life. Most notably was John Cassian, who wrote in his Conferences:
And so the grace of God always co-operates with our will for its advantage, and in all things assists, protects, and defends it, in such a way as sometimes even to require and look for some efforts of good will from it that it may not appear to confer its gifts on one who is asleep or relaxed in sluggish ease, as it seeks opportunities to show that as the torpor of man’s sluggishness is shaken off its bounty is not unreasonable, when it bestows it on account of some desire and efforts to gain it. And none the less does God’s grace continue to be free grace while in return for some small and trivial efforts it bestows with priceless bounty such glory of immortality, and such gifts of eternal bliss.2
Cassian enumerated a number of Scriptures in support of his opinion in the same Conferences:
Whence human reason cannot easily decide how the Lord gives to those that ask, is found by those that seek, and opens to those that knock, and on the other hand is found by those that sought Him not, appears openly among those who asked not for Him, and all the day long stretches forth His hands to an unbelieving and gainsaying people, calls those who resist and stand afar off, draws men against their will to salvation, takes away from those who want to sin the faculty of carrying out their desire, in His goodness stands in the way of those who are rushing into wickedness. But who can easily see how it is that the completion of our salvation is assigned to our own will, of which it is said: If ye be willing, and hearken unto Me, ye shall eat the good things of the land, (Isaiah 1:19) and how it is not of him that willeth or runneth, but of God that hath mercy? (Romans 9:16) What too is this, that God will render to every man according to his works; (Romans 2:6) and it is God who worketh in you both to will and to do, of His good pleasure; (Philippians 2:13) and this is not of yourselves but it is the gift of God: not of works, that no man may boast? (Ephesians 2:8-9) What is this too which is said: Draw near to the Lord, and He will draw near to you, (James 4:8) and what He says elsewhere: No man cometh unto Me except the Father who sent Me draw Him? (John 6:44) What is it that we find: Make straight paths for your feet and direct your ways, (Proverbs 4:26 LXX) and what is it that we say in our prayers: Direct my way in Thy sight, and establish my goings in Thy paths, that my footsteps be not moved? (Psalm 5:9) What is it again that we are admonished: Make you a new heart and a new spirit, (Ezekiel 18:31) and what is this which is promised to us: I will
give them one heart and will put a new spirit within them: and I will take away the stony heart from their flesh and will give them an heart of flesh that they may walk in Thy statutes and keep My judgments? (Ezekiel 1:19-20) What is it that the Lord commands, where He says: Wash thine heart of iniquity, O Jerusalem, that thou mayest be saved, (Jeremiah 4:14) and what is it that the prophet asks for from the Lord, when he says Create in me a clean heart, O God, and again: Thou shalt wash me, and I shall be whiter than snow? (Psalm 50:12 LXX). What is it that is said to us: Enlighten yourselves with the light of knowledge; (Hosea 10:12 LXX) and this which is said of God: Who teacheth man knowledge; (Psalm 93:10 LXX) and: the Lord enlightens the blind, (Psalm 145:8 LXX) or at any rate this, which we say in our prayers with the prophet: Lighten mine eyes that I sleep not in death, (Psalm 12:4 LXX) unless in all these there is a declaration of the grace of God and the freedom of our will.3
Some have painted Cassian's view of synergy - the cooperation of man's will with God's grace - with a broad brush and called it just Pelagianism, plain and simple. But belief that man is saved through the cooperation of his free will with God's grace is a far cry from belief that man can be saved through his free will alone. Even the synergistic view - which John Cassian points out has firm support in Scripture - is derided by some as being "semi-Pelagian".
There is nothing anywhere in the writings of John Chrysostom that suggests he holds Pelagius' views that man can be saved without God's grace. One need only look to his commentary on 1 Corinthians 15:10 to confirm this:
And His grace in me was not in vain; but I labored more abundantly than they all, and yet not I, but the grace of God with me
Seest thou again another excess of humility? In that the defects he imputes to himself, but of the good deeds nothing; rather he refers all to God. Next, lest he might hereby render his hearer supine, he saith, And His grace which was bestowed upon me was not found vain. And this again with reserve: in that he said not, I have displayed a diligence worthy of His grace, but, it was not found vain.4
1. In The Place of Blessed Augustine in the Orthodox Church, p.37.
3. Ibid., XIII.9
4. Homily XXXVIII on First Corinthians