The danger in adopting pagan cultural practices is that they have always tended to push out godly ones. This is what happened with the early church regarding the holidays. If you actually look closely at the New Testament scriptures, while they were not being kept as a law, nor after the strictness of the Jews, early church Christians were nevertheless keeping some of the Jewish holidays after a spiritual manner. Not many realize it because it isn't taught, but this was true of:
1. Passover, and the Feast of Unleavened Bread:
Clean out the old leaven so that you may be a new lump, just as you are in fact unleavened. For Christ our Passover also has been sacrificed. Therefore let us celebrate the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth (1 Corinthians 5:7-8.
See notes below*)
2. Pentecost:
For Paul had decided to sail past Ephesus so that he would not have to spend time in Asia. For he was hurrying to be in Jerusalem, if possible, on the day of Pentecost. (Acts 20:16. See notes below)**
Now concerning the collection for the saints... when I come, whomsoever you shall approve by letters, them will I send to bring your liberality to Jerusalem... But I will remain in Ephesus until Pentecost. (1 Corinthians 16:1, 3, 8. See notes below)***
I won't slow up this post by giving evidences for why these verses suggest the church was keeping these things. I will just add some things in the notes. But the question is what happened? Certainly the church doesn't still keep these observances today, so why not? According to Tertullian, it was because pagan culture crept in, and they eventually abandoned them in favor of the pagan festivals instead.
Writing around the mid 3rd century, he stated the following:
By us, to whom Sabbaths and the New Moons and festivals formerly beloved by God are [now] strange, the Saturnalia and New-year's and Midwinter's festivals and Matronalia are [instead] frequented - presents come and go - New-year's gifts - games join their noise - banquets join their din! Oh, what better fidelity of the nations to their own sect, which claim no solemnity of the Christians for itself! Not the Lord's day, not Pentecost, even if they had known them, would they have shared with us, for they would fear lest they should seem to be Christians. But we are not apprehensive lest we seem to be heathens! (Tertullian, from On Idolatry)
In other words, he was saying the nations would have NEVER kept festivals like Pentecost with the Christians lest they seem weird, but the church now had no problem keeping pagan festivals instead. And we still observe New Years and Christianized holidays like Christmas and Easter today. This won't send a Christian to Hell, of course, but when we abandoned the spiritual observance of the Jewish festivals after the first few centuries, we abandoned festivals that actually had Christian meaning behind them when understood prophetically. Thus, we abandoned meditation upon Christian truths, and upon prophecies that were being fulfilled during New Testament times, as well as others that have not yet come to pass.
______________
* The sentence that reads "Clean out the old leaven so that you may be a new lump, just as you are in fact unleavened" is hard to make out unless one understands how the Feast of Unleavened Bread was literally observed by the Jews. In Exodus, a description is provided of how God wanted the nation to observe it. They were to remove leaven completely from their houses during the seven day period:
On the first day you shall remove leaven out of your houses, for if anyone eats what is leavened from the first day until the seventh day, that person shall be cut off from Israel... For seven days no leaven is to be found in your houses. If anyone eats what is leavened, that person will be cut off from the congregation of Israel, whether he is a sojourner or a native of the land. You shall eat nothing leavened. In all your dwelling places you shall eat only unleavened bread.” (Exodus 12:15-19-20)
There were Jewish Christians present in Gentile congregations like at Corinth, and these would have kept this observance quite literally. Thus, it appears that the phrase "just as you are in fact unleavened" was a reference to the entire congregation both Jew and Gentile removing leaven entirely from their houses at this time, in remembrance of the Jewish feast. Paul's displeasure, however, was that they were not keeping it in a spiritual sense. To keep the feast spiritually, they needed to remove false teaching from among them, which is what Jesus said the leaven represented (
Matthew 16:6). They were clearly not doing so. One young man was "leavening the whole lump" by the immoral lifestyle he was living among them, thus spreading "leaven" throughout the congregation by the example he was setting (
1 Corinthians 5:1-5).
The Feast of Unleavened Bread is also mentioned in Acts 20:6 and Acts 12:3-4, and without any further elaboration this suggests the Gentile Christians needed none, and knew exactly what this Jewish Feast was and when it was held. Thus, based upon the texts in Acts and 1st Corinthians, it appears the Gentile church not only understood what Passover and the Feast of Unleavened Bread were, but observed them along with the Jewish Christians among them who would have had no reason not to. But again, the teaching was that no one needed to after the strict manner of the Jews now, for they were merely shadows of coming things.
**/*** Why would Paul have been hurrying to be in Jerusalem specifically for Pentecost if he was not desiring to attend it? It means that the apostle Paul kept Pentecost. And if so, wishing to "remain in Ephesus until Pentecost" suggests he was keeping it there as well. And if the apostle Paul was keeping it, the rest of the congregation likely honored it as well, even if only after a spiritual manner.