Why do Catholics believe many concepts that are not in the Bible – like assumption of Mary, child baptism etc?

The first step towards knowing what a Christian needs to believe would be to gain the understanding of what Jesus and his apostles along with the various New Testament (NT) writers put forth as the rule of faith. Is it just the written Word of God or is there something else? 

The first glimpse of what the Early Church took as the rule of faith comes from the second chapter of the Acts – “They devoted themselves to the apostles’ teaching…” (Acts 2:42). Keep in mind that at this point of time there simply was no New Testament as the first book wouldn’t be written only after 50 AD. From this it is clear that the fullness of Christian teaching was found in the Church. Even after the NT books were written, we find that still those books were not considered as the complete rule of faith. This can be seen in the admonitions given by the NT writers to the believers where they ask them to “stand firm and hold fast to the traditions” which were handed to them “either by word of mouth or by our letter” (2 Thessalonians 2:15). Again in 2 Timothy 2:2 we see that much of Christian teaching is to be found in the oral teachings of the apostles – “what you have heard from me through many witnesses” – and that the teachings thus obtained was to be handed down to those who will teach others – “entrust to faithful people who will be able to teach others as well”. 

This oral teaching was considered by the early Chritians to be on par with the written Word of God that they received later from the NT writers and why wouldn’t they as it was Jesus himself who had given the command – “Whoever listens to you listens to me, and whoever rejects you rejects me, and whoever rejects me rejects the one who sent me” (Luke 10:16). Paul too speaks about the importance of the oral teaching when he says – “So faith comes from what is heard, and what is heard comes through the word of Christ” (Romans 10:17). Hear from whom? To whom was the teaching authority given? The answer is simple: the apostles.

The term apostle is derived from Classical Greek ἀπόστολος (apóstolos), meaning “one who is sent off”. The purpose of such sending off is usually to convey a message, and thus “messenger” is a common alternative translation. It is for this purpose, to be the messengers of Christ, that Jesus promises the Holy Spirit to the apostles, who would teach them all things and guide them into all the truth (John 14:16, 26; 16:13). He then gives them (the apostles) the command to go and make disciples, baptise them and teach them (Matthew 28:18-20).

2 Thessalonians 2:15, 2 Thessalonians 2:14-15, Hebrews 13:7-9 and Hebrews 13:17 are just some of the verses that speak about the authority of the apostles. 

It can also be seen that this authority was given to others by the apostles. For example Timothy, the first bishop of Ephesus, was instructed by Paul to “give attention to the public reading of scripture, exhorting to teaching” and “not neglect the gift that” was given to him “with the laying on of hands by the council of elders.” The apostles not only appointed others and gave them the authority to teach (Acts 14:23) but to some also gave authority to appoint still others (2 Timothy 2:2, Titus 1:5). This, giving teaching authority by the laying of hands, has continued to the present and forms the living teaching office of the Church. 

It is clear from all of this that:

1. The Written Scripture is not the only source of revelation given by God. 
2. Oral revelation received from Jesus and the apostles, called the Holy Tradition, is to be held on par with the Bible
3. The Holy Tradition transmits it to the successors of the apostles and is found in the Catholic Church (CCC 81)

As a result the Church,…“does not derive her certainty about all revealed truths from the holy Scriptures alone. Both Scripture and Tradition must be accepted and honoured with equal sentiments of devotion and reverence” (CCC 82). Moreover, the Bible does not support Scripture alone as “the” rule of faith.

Does this mean that the Assumption of Mary, child baptism and seemingly disregard of the commandment regarding idols do not have any Scriptural basis? Let’s delve into the Scripture and find it for ourselves. 

“You shall not make for yourself an idol, whether in the form of anything that is in heaven above, or that is on the earth beneath, or that is in the water under the earth.” Exodus 20:4

What is an idol in the biblical sense? The answer lies in the very next verse – “You shall not bow down to them or worship them…” – Exodus 20:5. An idol is something which takes the place of God in our lives. The Catholic Church understands this verse completely and says – Idolatry not only refers to false pagan worship. It remains a constant temptation to faith. Idolatry consists in divinizing what is not God. Man commits idolatry whenever he honors and reveres a creature in place of God, whether this be gods or demons (for example, satanism), power, pleasure, race, ancestors, the state, money, etc. Jesus says, “You cannot serve God and mammon.” – CCC 2113

Then why is it that we find so many images and statues in the Catholic Church. For this we will have to look into Exodus 25:18-20 where God himself asks to make a statue of two cherubims adorning the mercy seat. Not only this God also commanded Moses to make a bronze serpent so that “everyone who is bitten shall look at it and live” (Numbers 21:8). Was God contradicting himself? Of Course not. In Exodus 20:5 he was talking about “idols” and in the later verses mentioned here, God is talking about “statues”. As said earlier an idol replaces God; while a statue or an image when used properly as mentioned in the scripture, draws one towards God. This is made clear when we see that it pleased God when Hezekiah destroyed the bronze serpent made by Moses. Why? Because “until those days the people of Israel had made offerings to it” (2 Kings 18:4) – thereby changing a symbol of God’s mercy (a statue) into an idol. 

The statues and images in the Church are mere things which act similar to road signs, ultimately directing one towards the maker of all – God – and by no means are gods themselves. Therefore the Church teaches thus:

The Christian veneration of images is not contrary to the first commandment which proscribes idols. Indeed, “the honor rendered to an image passes to its prototype,” and “whoever venerates an image venerates the person portrayed in it.” The honor paid to sacred images is a “respectful veneration,” not the adoration due to God alone: Religious worship is not directed to images in themselves, considered as mere things, but under their distinctive aspect as images leading us on to God incarnate. The movement toward the image does not terminate in it as an image, but tends toward that whose image it is. – CCC 2132

Read also:
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Why do Catholic Church believe in the assumption of Mary