Does synergism imply weakness of God?

See also Synergism - Monergism.

Calvinism claims that if God wants all to be saved, literally all people, and man can choose to reject salvation, then man is overpowering God.

However, by this line of reasoning, Peter was right in trying to forbid man from overpowering God in crucifying the Messiah! But Jesus did not see it this way, in Matt 16:23. 1

But, we might argue, Peter was going against God's plan. 

Yes, and even though God incarnate was to be humiliated, shamed, and killed - and even though Peter was arguably just seeking to preserve the glory of God - Peter was wrong, because the cross was God's plan. 

This is strikingly parallel to God's plan for salvation. Calvinists say man's ability to reject salvation is contrary to the glory or sovereignty of God, and thus not possible. However, the identical reasoning applies to the case of Peter, that the cross was man overpowering God, and we have a double standard. 

The claim that it is God's plan to save those that believe, seems insufficient  to Calvinists. Then tell me why it is sufficient in the case of Peter?

The argument used by Calvinists to refute free choice to receive Christ is even weaker than in the case of Peter, where it does not apply. So why does it apply at all, in a case where the diminution of God's glory, sovereignty, etc. are all less? and not where this diminution is greater?

For which is a greater blow to God's sovereignty, power, and glory - the humiliation and death of God incarnate, or the simple choice of man to receive or reject something offered to him by God?

Obviously, the argument Calvinists use here against man's free will is much more strongly supportive of Peter in preventing Christ from going to the cross, than it is against man's free will. But Peter was wrong to try to stop the atonement, and Calvinists are also wrong in this.

What is the criterion for application of this double standard? It is not scripture. It is some philosophical, ideological presupposition brought to the scripture in eisegesis.

Summary

To Peter, the cross likely was man overpowering God.

To Calvinists, man's ability to reject salvation also is man overpowering God.

The cross was justified because the cross was God's will.

Man's ability to reject salvation is justified because man's ability to reject is also God's will. (It pleased God to save those that believe.)

These 2 stand and fall together.

If God's sovereignty trumps God's will, then both the cross and man's will are refuted.

If God's will trumps God's sovereignty, then both the cross and man's will are confirmed.

The question of whether sovereignty requires micro-manipulation which by definition precludes free will is dealt with here. 

• 1Matthew 16:23 - 
  But he turned, and said unto Peter, Get thee behind me, Satan: thou art an offence unto me: for thou savourest not the things that be of God, but those that be of men.