The Second Helvetic Confession (1566)  states (underlining mine):
“THE TEACHING OF THE GOSPEL IS NOT NEW, BUT MOST ANCIENT DOCTRINE.
And although the teaching of the Gospel, compared with the teaching of the Pharisees concerning the law, seemed to be a new doctrine when first preached by Christ (which Jeremiah also prophesied concerning the New Testament), yet actually it not only was and still is an old doctrine (even if today it is called new by the Papists when compared with the teaching now received among them), but is the most ancient of all in the world. For God predestinated from eternity to save the world through Christ, and he has disclosed to the world through the Gospel this his predestination and eternal counsel (II Tim. 1:9-10). Hence it is evident that the religion and teaching of the Gospel among all who ever were, are and will be, is the most ancient of all. Wherefore we assert that all who say that the religion and teaching of the Gospel is a newly concocted faith, being scarcely thirty years old, err disgracefully and speak shamefully of the eternal counsel of God. To them applies the saying of Isaiah the prophet: ‘Woe to those who call evil good and good evil, who put darkness for light and light for darkness, who put bitter for sweet and sweet for bitter!’ (Isa. 5:20)” (Second Helvetic Confession).
This is not to set forth the entire contents of this Confession as promulgating and promoting the true gospel, but to simply say that a typical tolerant Calvinist characteristic (or penchant) is to disgracefully disparage the true gospel as something quite novel.
Isaiah 5:20 forcefully epitomizes the tolerant religionist mindset (religionists of varying stripe, but especially and primarily those calling themselves “Christian”). The idea (some might call it a “charitable” and “irenical” idea) conveyed is that “truly regenerate Christians” begin their spiritual lives in darkness and then “progress” or “grow into” light (contra Romans 10:1-4). Thus, theirs is a pitch black progress that is ever-learning but never able to acknowledge the truth in Jesus (cf. Ephesians 4:18-21); a beginning with and savoring the bitter and a spewing and spitting out of the sweet.
“The stone [which] the builders refused is become the head [stone] of the corner. This is the LORD’S doing; it [is] marvellous in our eyes” (Psalm 118:22-23).
We are presenting this stone as THE essential core and foundation. Your typical Calvinist connoisseur or mature and balanced Reformed person REFUSE such a stone as this since that would be a cultic, autocratic, unbalanced, unloving, and imperious demand for doctrinal perfectionism (or some other nonsensical variant). Our response to accusations as ancient as they are threadbare, is not to tilt toward antipathy, fall into despondency, or slide into apathy, but to cry out to the Triune God of Scripture, we are insufficient for these things!!
“But we had the sentence of death in ourselves, that we should not trust in ourselves, but in God which raiseth the dead: Who delivered us from so great a death, and doth deliver: in whom we trust that he will yet deliver [us]” (2 Corinthians 1:9-10).
“Now thanks [be] unto God, which always causeth us to triumph in Christ, and maketh manifest the savour of his knowledge by us in every place. For we are unto God a sweet savour of Christ, in them that are saved, and in them that perish: To the one [we are] the savour of death unto death; and to the other the savour of life unto life. And who [is] sufficient for these things?” (2 Corinthians 2:14-16)
“Not that we are sufficient of ourselves to think any thing as of ourselves; but our sufficiency [is] of God” (2 Corinthians 3:5).
And further to emulate Paul’s proclamation here:
“Having therefore obtained help of God, I continue unto this day, witnessing both to small and great, saying none other things than those which the prophets and Moses did say should come” (Acts 26:22).
Prior to his conversion, and despite having been well-versed in the prophets and Moses, Paul was woefully ignorant regarding Christ as the end of the law for righteousness (Romans 10:1-4). So what happened? 2 Corinthians 4:6 happened.
“Nicodemus answered and said unto him, How can these things be? Jesus answered and said unto him, Art thou a master of Israel, and knowest not these things?” (John 3:9-10)
This passage is striking and instructive. A master of Israel cannot see the kingdom of God unless he is regenerated by the Spirit of God (John 3:5-6; cf. 2 Corinthians 4:3-7).
“I am verily a man [which am] a Jew, born in Tarsus, [a city] in Cilicia, yet brought up in this city at the feet of Gamaliel, [and] taught according to the perfect manner of the law of the fathers, and was zealous toward God, as ye all are this day. And I persecuted this way unto the death, binding and delivering into prisons both men and women” (Acts 22:3-4; cf. Philippians 3:1-9; underlining mine).
Just because Nicodemus and Gamaliel (and most of Israel) did not see in Scripture the brightness of the gospel of the glory of Christ, it does not mean that it was not always there (2 Corinthians 4:3-6; cf. Romans 9:6-8). It is no cogent argument that Christ’s redemptive glory being seen is novel, just because many do not see it (2 Corinthians 4:3-4; cf. Matthew 11:25-27).
“Therefore they could not believe, because that Isaiah said again, He hath blinded their eyes, and hardened their heart; that they should not see with [their] eyes, nor understand with [their] heart, and be converted, and I should heal them. These things said Isaiah, when he saw His glory, and spake of Him” (John 12:39-41).
 Schaff writes concerning this confession:
“It is the work of Henry [or Heinrich–CD] Bullinger (1504–1575), the pupil, friend, and successor of Zwingli, to whom he stands related as Beza does to Calvin…Bullinger was one of the principal authors of the First Helvetic Confession, and the sole author of the Second…The Helvetic Confession is the most widely adopted, and hence the most authoritative of all the Continental Reformed symbols, with the exception of the Heidelberg Catechism.”