I find it interesting that you consider Easter pagan. Have you done the research into this? Do you know what Easter is called everywhere else in the world except Germany and England? It is called Pascha, which obviously derives from the Hebrew Passover and we see the practice firmly established in the second century. If you read a book titled “On pascha” by Saint Melito of Sardis who was a jew I might add, what he does in his explanation of the exodus is to turn the focus from the old and useless passover (In which the jews would sacrifice a lamb) to the new passover in Jesus Christ. And Melito was a Jew, yet he did not merely practice the same old traditions, he with the other Christians set everything on Christ and hence the worship of the old pascha was transformed into the new. Easter is not pagan, but rather a natural apostolic outgrowth which I believe started with the apostles. I find it impossible to imagine the apostles worship didn’t change at all, that they did not reflect on their practice of pascha and feel the need to bring Christ into the center of it instead of merely the old exodus celebration.
Also, sunday worship existed before constantine.
Consider this early Christian epistle, although not from Barnabas it does represent the early church.
Wherefore also we keep the eighth day for rejoicing, in the which also Jesus rose from the dead, and having been manifested ascended into the heavens.
There is also what Ignatius says.
Be not seduced by strange doctrines nor by antiquated fables, which are profitless. For if even unto this day we live after the manner of Judaism, we avow that we have not received grace … If then those who had walked in ancient practices attained unto newness of hope, no longer observing Sabbaths but fashioning their lives after the Lord’s day, on which our life also arose through Him and through His death which some men deny … how shall we be able to live apart from Him? … It is monstrous to talk of Jesus Christ and to practise Judaism. For Christianity did not believe in Judaism, but Judaism in Christianity — Ignatius to the Magnesians 8:1, 9:1-2, 10:3, Lightfoot translation.
I think were messianicism errs is that its worship has not been transformed by Christ, it keeps all the old things without rein orientating them towards their fulfillment which the Christian feasts and fasts appropriately do. None of us are bound to keep the Sabbath, to be circumcised or keep the old law as the apostle Paul told us very clearly. We are free in Christ Jesus.