Consider the testimony of Scripture itself. 1Corinthians 15:3-19
1 Corinthians 15:3–19 (NASB95) — 3 For I delivered to you as of first
importance what I also received, that Christ died for our sins
according to the Scriptures, 4 and that He was buried, and that He was
raised on the third day according to the Scriptures, 5 and that He
appeared to Cephas, then to the twelve. 6 After that He appeared to
more than five hundred brethren at one time, most of whom remain until
now, but some have fallen asleep; 7 then He appeared to James, then to
all the apostles; 8 and last of all, as to one untimely born, He
appeared to me also. 9 For I am the least of the apostles, and not fit
to be called an apostle, because I persecuted the church of God. 10
But by the grace of God I am what I am, and His grace toward me did
not prove vain; but I labored even more than all of them, yet not I,
but the grace of God with me. 11 Whether then it was I or they, so we
preach and so you believed. 12 Now if Christ is preached, that He has
been raised from the dead, how do some among you say that there is no
resurrection of the dead? 13 But if there is no resurrection of the
dead, not even Christ has been raised; 14 and if Christ has not been
raised, then our preaching is vain, your faith also is vain. 15
Moreover we are even found to be false witnesses of God, because we
testified against God that He raised Christ, whom He did not raise, if
in fact the dead are not raised. 16 For if the dead are not raised,
not even Christ has been raised; 17 and if Christ has not been raised,
your faith is worthless; you are still in your sins. 18 Then those
also who have fallen asleep in Christ have perished. 19 If we have
hoped in Christ in this life only, we are of all men most to be
pitied.
The testimony of Scripture to this age-old question is centered around the resurrection of Jesus Christ. The entire Christian faith relies upon the reality and truthfulness of Jesus Christ, who is God, lived a sinless life (Heb 4:15; 1Peter 2:22; 2Corinthians 5:21; 1John 3:5), died and was buried, and rose on the third day according to the Scriptures (Hosea 6:2; Luke 24:46).
Why did Christianity "take off"? That question has been answered many different ways throughout history. Probably the most helpful and memorable is the one offered by C.S. Lewis who said:
I am trying here to prevent anyone saying the really foolish thing
that people often say about Him: I’m ready to accept Jesus as a great
moral teacher, but I don’t accept his claim to be God. That is the one
thing we must not say. A man who was merely a man and said the sort of
things Jesus said would not be a great moral teacher. He would either
be a lunatic—on the level with the man who says he is a poached egg—or
else he would be the Devil of Hell. You must make your choice. Either
this man was, and is, the Son of God, or else a madman or something
worse. You can shut him up for a fool, you can spit at him and kill
him as a demon or you can fall at his feet and call him Lord and God,
but let us not come with any patronising nonsense about his being a
great human teacher. He has not left that open to us. He did not
intend to. . . . Now it seems to me obvious that He was neither a
lunatic nor a fiend: and consequently, however strange or terrifying
or unlikely it may seem, I have to accept the view that He was and is
God. (Mere Christianity, 55-56)
His argument has been whittled down to Jesus as one of three possiblities: Liar, Lunatic, or Lord. But the conclusion of so many eye-witnesses was that He was indeed Lord. For example the Centurion soldier:
Matthew 27:54 (NASB95) — 54 Now the centurion, and those who were with
him keeping guard over Jesus, when they saw the earthquake and the
things that were happening, became very frightened and said, “Truly
this was the Son of God!”
So to answer your question, Christianity has been tested since the beginning. The Jews tried to discredit Jesus, the Pharisees and Saduccees tried to trap him, Judas tried to kill Him, the Romans tried to judge Him, but Jesus had full control of His life as he would often say, "My time has not yet come" (John 7:6). Ultimately it was the Father who crushed the Son (Isaiah 53:10). Jesus gave up His life of His own accord because He was God (Matthew 27:50).
The arguments for atheism are fairly recent. Even the wicked pagans and Jews believed in God. At least they were not as foolish as the people of today who believe there is no God (Psalm 53:1). The attacks on Christianity was to discredit Jesus, to expose Him as a fraud, to retain power over the region of Judea, to preserve the social structure of the Jews. Yet the answer for all is the same. Jesus Christ who lived, died, was raised on the third-day, seated on the right-hand of the Father remains the same answer yesterday, today, and forever.