"The ancient Christians were animated by a contempt for their present existence, and by a just confidence of immortality." Very true; but the fact of their being so animated was what wanted explaining.

Gibbon says it "was no wonder that so advantageous an offer" as that of immortality was accepted.

Yet he had just before told us that the ablest orators at the bar and in the senate of Rome, could expose this offer of immortality to ridicule without fear of giving offence.
Whence arose, then, the sudden blaze of conviction with which the Christians embraced it?
The third cause is the miraculous powers _ascribed_ to the primitive Church.

Gibbon apparently had not the courage to admit that he agreed with his friend Hume in rejecting miracles altogether.

He conceals his drift in a cloud of words, suggesting indirectly with innuendo and sneer his real opinion.