BASICS—THE FUNDAMENTALS

"WHICH IN OTHER AGES WAS NOT MADE KNOWN TO THE SONS OF MEN, AS IT HAS NOW BEEN REVEALED BY THE SPIRIT TO HIS HOLY APOSTLES AND PROPHETS."
Ephesians 3:5 (NKJV)

Comment: The old testament saints and the early Jewish believers did not fully understand the redemption plan of God in which He intended to incorporate the gentiles, bring them into His family (Acts 15:15-18; Ephesians 2:12,13,19). They had the thought that salvation through Christ Jesus would be exclusive for the Jews (Acts 10:45; 11:1-3). And If at all any other nations of the world wanted to come in, It has to be through the procedure of proselytization or conversion, in which the person would be circumcised and be introduced to the practicing of the old testament rituals or pattern of worship: "AND CERTAIN MEN CAME DOWN FROM JUDEA AND TAUGHT THE BRETHREN, “UNLESS YOU ARE CIRCUMCISED ACCORDING TO THE CUSTOM OF MOSES, YOU CANNOT BE SAVED" (Acts 15:1 NKJV).
They wanted whoever is saved to be observing days, practicing the dos and don'ts in the old testament which cannot bring a change or transformation (Colossians 2:21-23; Hebrews 10:1).

Whatever was given to them in the old testament to be practicing was a shadow of things to come: "THE OLD SYSTEM UNDER THE LAW OF MOSES WAS ONLY A SHADOW, A DIM PREVIEW OF THE GOOD THINGS TO COME, NOT THE GOOD THINGS THEMSELVES. THE SACRIFICES UNDER THAT SYSTEM WERE REPEATED AGAIN AND AGAIN, YEAR AFTER YEAR, BUT THEY WERE NEVER ABLE TO PROVIDE PERFECT CLEANSING FOR THOSE WHO CAME TO WORSHIP" (Hebrews 10:1 NLT).
READ Colossians 2:17
They were to be doing those things, works of the law, to keep them in check and prepare them for the coming of the Messiah—faith in Christ Jesus (Galatians 3:23,24).
Now that Christ has come, whoever places his or her faith in Him is accepted by God, thus, there is no more need for the law and its works or rituals (Acts 10:34,35; Galatians 3:25).
Even the early believers, including the Apostles, did not fully understand this mystery, the infinite wisdom of God—His variegated wisdom (Ephesians 3:10). This mystery was revealed by the Holy Spirit, as Jesus did promise before ascension (John 16:12-15). It was revealed by the Holy Spirit to His Apostles and prophets (Ephesians 3:5). This mystery is being contested today by those who are ignorant and lack the understanding of the Scriptures (2 Peter 3:15,16).
Note:
a. Faith in the substitutionary death and resurrection of Christ Jesus is enough for the salvation of whoever wants to access or reach God (John 14:6; Romans 10:9-13).
b. You would not need any additional works or rituals to do for you to be reconciled to God (Galatians 2:16; 2 Corinthians 5:18).
These basic tenets of faith had to be understood by every believer in Christ Jesus so that you would not be misguided or misled to some cults that asserted or claimed to be Christians.
Peace.