Wesley and Whitefield were initially both members of the Church of England
Thirty-Nine Articles of Faith, Article XVII:
Predestination to Life is the everlasting purpose of God, whereby (before the foundations of the world were laid) he hath constantly decreed by his counsel secret to us, to deliver from curse and damnation those whom he hath chosen in Christ out of mankind, and to bring them by Christ to everlasting salvation, as vessels made to honour …
The godly consideration of Predestination, and our Election in Christ, is full of sweet, pleasant, and unspeakable comfort to godly persons, and such as feel in themselves the working of the Spirit of Christ, mortifying the works of the flesh, and their earthly members, and drawing up their mind to high and heavenly things, as well because it doth greatly establish and confirm their faith of eternal Salvation to be enjoyed through Christ, as because it doth fervently kindle their love towards God.
John Wesley
Sermon 128, “Free Grace” on Romans 8:32, at Bristol, April 29 1739
https://www.ccel.org/ccel/wesley/sermons/sermons-html/serm-128.html
(Predestination) is plainly this—by virtue of an eternal, unchangeable, irresistible decree of God, one part of mankind are infallibly saved, and the rest infallibly damned; it being impossible that any of the former should be damned, or that any of the latter should be saved. But if this be so, then is all preaching vain. It is needless to them that are elected; for they, whether with preaching or without, will infallibly be saved …
Let it be observed, that this doctrine represents our blessed Lord, ‘Jesus Christ the righteous,’ ‘the only begotten Son of the Father, full of grace and truth,’ as an hypocrite, a deceiver of the people, a man void of common sincerity. For it cannot be denied, that he everywhere speaks as if he was willing that all men should be saved …
And here I fix my foot. On this I join issue with every assertor of [a Calvinistic interpretation of the doctrine of predestination]. You represent God as worse than the devil; more false, more cruel, more unjust … Sing, O hell, and rejoice, ye that are under the earth! For God, even the mighty God, hath spoken, and devoted to death thousands of souls, from the rising of the sun unto the going down thereof!… Let all the sons of hell shout for joy!
Explanatory Notes Upon the New Testament, 1755, Wesley comments on the word ‘elect’ in 1 Peter 1:2, that:
The true predestination, or fore-appointment of God is,
1. He that believeth shall be saved from the guilt and power of sin.
2. He that endureth to the end shall be saved eternally.
3. They who receive the precious gift of faith, thereby become the sons of God; and, being sons, they shall receive the Spirit of holiness to walk as Christ also walked.
George Whitefield responded in a letter “Answer to Free-Grace”
https://www.gracegems.org/33/whitefields_letter_to_wesley.htm
I believe the Doctrine of Reprobation (17th Article):
“That God intends to give saving grace through Jesus Christ only to a certain number; and that the rest of mankind, after the fall of Adam, being justly left by God to continue in sin—will at last suffer that eternal death which is its proper wages.”
Has not God, who has appointed salvation for a certain number, also appointed the preaching of the Word as a means to bring them to it? Does anyone hold election in any other sense? And if so, how is preaching needless for those who are elected, when the gospel is designated by God himself to be the power of God unto their eternal salvation? (Romans 1:16) And since we do not know who are elect and who are reprobate, we are to preach promiscuously to all.
Without the belief of the doctrine of election, and the immutability of the free love of God, I cannot see how it is possible that anyone should have a comfortable assurance of eternal salvation. What could it signify to a man whose conscience is thoroughly awakened, and who is warned in good earnest to seek deliverance from the wrath to come, even though he should be assured that all his past sins are forgiven, and that he is now a child of God—if notwithstanding this, he may afterward become a child of the devil, and be thrown into Hell at last? Could such an assurance yield any solid, lasting comfort to a person convinced of the corruption and treachery of his own heart, and of the malice, subtlety, and power of Satan?
Blessed be God, our Lord knew for whom he died. (2 Timothy 2:19) There was an eternal compact between the Father and the Son. A certain number was then given him as the purchase and reward for his obedience and death. He prayed for these, and not for the world (John 17:9). For these elect ones, and these only, he is now interceding, and with their salvation he will be fully satisfied.
George Whitefield George Whitefield: The Life and Times of the Great Evangelist of the 18th Century Revival, Vol. 1 Banner of Truth Trust; 1970, p. 407.
Man is nothing; He hath a Free Will to go to hell, but none to go to heaven, till God worketh in him to will and to do His good pleasure.
Wesley vs. Whitefield
https://christianhistoryinstitute.org/magazine/article/wesley-vs-whitefield
Wesley and Simeon
https://www.desiringgod.org/articles/a-250-yr-old-model-how-calvinist-simeon-related-to-wesley
http://www.calvinism.us/2020/09/charles-simeon-1759-1836.html
Spurgeon's defense of Calvinism, “Exposition of the Doctrine of Grace”, The New Park Street and Metropolitan Tabernacle Pulpit, “Opening Meetings of Metropolitan Tabernacle”, April 11, 1861
https://www.spurgeongems.org/sermon/chs364A.pdf
The controversy which has been carried on between the Calvinist and the Arminian is exceedingly important, but it does not so involve the vital point of personal godliness as to make eternal life depend upon our holding either system of theology. I think we are free to admit, that while John Wesley, for instance, in modern times zealously defended Arminianism, and on the other hand, George Whitfield with equal fervour fought for Calvinism, we should not be prepared either of us, on either side of the question, to deny the vital godliness of either the one or the other.