The phrase to rush in where angels fear to tread means: to embark enthusiastically on a course of action that most sensible people would avoid.

This phrase occurs, for example, in This Week in… Homes & Property, by Janice Morley, published in the Evening Standard (London, England) of Wednesday 22^nd August 2018:

SUMMER is almost over, the school holidays are coming to an end and you are having to drag yourself out of the sun, off the sand and back home to routine school runs, commuter chaos—and winter. It’s not a great prospect is it? But what can you do? Well Kieran and Helen Fisher found the cure for back-to-work blues—they didn’t go back. They sold up and, for a modest amount, bought a 300-year-old cottage with 12 acres in North Devon where they put phase one of their new plan into action. They built a fabulous tree house to let to paying guests. Phase two is about to start. Find out about their journey on Page 8, be inspired to go where angels fear to tread and you might even be able to exploit a tax loophole along the way: an added bonus for being bold.

The phrase was coined as fools rush in where angels fear to tread by the English poet Alexander Pope (1688-1744) in An Essay on Criticism (London: Printed for W. Lewis […]; And Sold by W. Taylor […], T. Osborn […], and J. Graves […]. MDCCXI [1711]):

Such shameless Bards we have; and yet ’tis true,
There are as mad, abandon’d Criticks too.
The Bookful Blockhead, ignorantly read,
With Loads of Learned Lumber in his Head,
With his own Tongue still edifies his Ears,
And always List’ning to Himself appears.
All Books he reads, and all he reads assails,
From Dryden’s Fables down to D——y’s Tales.
With him, most Authors steal their Works, or buy;
Garth did not write his own Dispensary.
Name a new Play, and he’s the Poet’s Friend,
Nay show’d his Faults—but when wou’d Poets mend?
No Place so Sacred from such Fops is barr’d,
Nor is Paul’s Church more safe than Paul’s Church-yard:
Nay, run to Altars; there they’ll talk you dead;
For Fools rush in where Angels fear to tread.

The earliest occurrences of the phrase to rush in where angels fear to tread that I have found are as follows, in chronological order:

1-: From the following dialogue between Bellamine and Melissa, in The Rival Modes. A Comedy (London: Printed for Bernard Lintot, 1727), by the English playwright James Moore Smythe (1702-1734):

Bell. Nay, fly to Altars there I’ll talk you dead.
Mel. For Fools rush in where Angels fear to tread.

2-: From A friendly Conference between Matter and Spirit, in the Characters of Somebody and Nobody: Being a compleat Illustration of a learned, elegant, and instructive Treatise, entitled, An Essay on Spirit (Dublin; Printed for Peter Wilson, 1752):

Nobody. It is hard to say which is most shocking, to be ludicrous or serious on surveying the temerity of a writer, who will venture to trifle with so tremendous a subject. What the angels have in vain desired to look into, he takes upon him to reveal. Be he the highest of mortal men, it is no want of respect to say, that “fools rush in, where angels fear to tread.” To create, is, in mortal apprehension, (if infinite can admit of degrees) the greatest of God’s incommunicable attributes, to imagine the power delegated to another is treason against the sovereign of the universe, and an attempt upon his crown and dignity.

3-: From Hezekiah: A Sacred Ode. Inscribed to the Benefactors of the Lying-In Hospital, in Dublin (Dublin: Printed by Richard James, 1756), by the Irish poet and playwright Henry Jones (1721-1770)—the phrases where angels stand afraid and where angels fear to tread occur in the following, about Israel’s God’s Temple:

The Chamber of his Presence fill’d with Glory
In Majesty divine array’d,
Where Angels stand afraid,
And where the purify’d high Priest alone
Can enter once a Year,
Almighty Anger to attone.
Shall horrid Sacrilege prophane our Story?
Shall heathen Footsteps enter there,
Where Angels fear to tread?

4-: From A View of Lord Bolingbroke’s Philosophy, compleat, in Four Letters to a Friend (London: Printed for A. Millar, and J. and R. Tonson, and S. Draper, 1756), by William Warburton (1698-1779), Bishop of Gloucester and religious controversialist:

In a word, the truth is no more than this, Presumptuous man knows not where to stop. He would penetrate even to the Arcana of the Godhead.
“For Fools rush in, where Angels fear to tread.”

5-: From Sermon II. 2 Cor. ii. 17. We are not as many, which corrupt the word of God; but as of sincerity, but as of God, in the sight of God, speak we in Christ., published in Three sermons preached before the University of Cambridge, occasioned by an attempt to abolish subscription to the XXXIX articles of religion (Cambridge: Printed by J. Archdeacon, Printer to the University, 1772), by Samuel Hallifax (1733-1790), successively Bishop of Gloucester and Bishop of St Asaph:

Such are the humble sentiments of unaffected piety on a question, in contemplation of which even Angels veil their faces, and which of all others is the least subject to the determinations of human reason. “But Fools rush in, where Angels fear to tread.” The profane levity and mockery, with which the doctrine of the adorable Trinity hath been treated by the scoffers of the present age, who would penetrate into the arcana of the Godhead, and fathom the depths of the Divine Nature with the short line of their own understanding, is known to all, who are in the least conversant in their writings.

6-: From A Review of a Free Enquiry into the Nature and Origin of Evil, published in Miscellaneous and Fugitive Pieces. Volume the First (London: Printed for T. Davies, 1774):

Surely a Man who seems not completely Master of his own Opinion, should have spoken more cautiously of Omnipotence, nor have presumed to say what it could perform, or what it could prevent. I am in doubt whether those who stand highest in the Scale of Beings speak thus confidently of the Dispensations of their Maker:
For Fools rush in, where Angels fear to tread.