What does it mean to be "weak in faith"?

Paul defines what being weak in the faith means for us. Its always best to use the bible to define bible terms, otherwise we are just making up our own definitions. First of all Paul is NOT critical of those who are 'weak in the faith', in contrast, he is critical of those who most would consider to be 'strong in the faith'. Rom 14:1 "Rom 14:1 "Receive one who is weak in the faith, but not to disputes over DOUBTFUL THINGS." Paul is dealing with a DOUBTFUL topic here, and it is whether we can eat meat or not. Paul does not say a person has less 'faith' or belief in God because they choose to be a vegetarian. He is not saying a person who chooses to be obedient to God as they understand it is 'weak in the faith'. There is no law against NOT eating meat. Rom 6:1-2 "What shall we say then? Shall we continue in sin that grace may abound? Certainly not!"

He is using that phrase to describe their knowledge of God's word. A new believer might consider that we need to avoid meat because it's been sacrificed to an idol prior to being sold in the butcher shop. God's word, His law, does not say you can't eat meat you buy if it's been sacrificed to an idol before the butcher bought it to sell to you. The prohibition is against participating in the sacrifice itself. If the meat has no blood in it (because it hasn't been strangled as per Acts 15:20) , it is fine to buy from the butcher. But to someone who thinks it is wrong, avoid eating that meat in front of them. If they want to abstain from meat altogether, there is no law against that either. it doesn't mean they have less 'faith' as in belief in God than you do. Pauls only criticism is for the meat eater, that they should show love to their vegetarian brother; Rom 14:15 "Yet if your brother is grieved because of your food, you are no longer walking in love". How do we become 'strong in the faith'? Paul tells us it is by hearing (hearing, learning and obeying) God's word. Rom 10:17 "So then faith comes by hearing, and hearing by the word of God". We need to hear God's word, so we can grow 'strong in faith' and be obedient to the faith Rom 1:5 "Through Him we have received grace and apostleship for OBEDIENCE to the faith"

Just another possible approach — Being “weak in faith” may not mean the same thing as having less faith than another.

Perhaps being “weak in faith” isn’t a measure of belief, but of spiritual strength in the moment. In some cases, simply feeling weakened by the burdens of what is going on in the world around us (relationships, etc.), though still filled with faith. One could be “weak”, but still be “in faith”.

1 Corinthians 1:27 (NKJV)
But God has chosen the foolish things of the world to put to shame the wise, and God has chosen the weak things of the world to put to shame the things which are mighty;

2 Corinthians 12:7-9 (NKJV)
⁷ And lest I should be exalted above measure by the abundance of the revelations, a thorn in the flesh was given to me, a messenger of Satan to buffet me, lest I be exalted above measure. ⁸ Concerning this thing I pleaded with the Lord three times that it might depart from me. ⁹ And He said to me, “My grace is sufficient for you, for My strength is made perfect in weakness.”

It seems that Paul was told that he was displaying weakness. Was this weakness - though still in faith – a sign that he had less faith than another? Or, was he being asked to rely on Christ for his strength.

Paul goes on to say in 2 Corinthians 12:10 (NKJV)

Therefore I take pleasure in infirmities, in reproaches, in needs, in persecutions in distresses, for Christ’s sake. For when I am weak, then I am strong.

In psychology, surveys to assess personal values or beliefs often make use of some kind of ratings scale delivered via a questionnaire, such as, "On a scale of 1 to 10, with 1 being totally uncertain and 10 being absolutely certain, how certain are you that the miracle of the loaves and the fishes occurred as described in the Bible?" (You could also have the scale extend to negative values to indicate that you actively don't believe a particular item.)

These sorts of questionnaires are currently used, in fact. One is the Santa Clara Strength of Religious Faith Questionnaire, which only has 10 questions.

Being "weak in faith" could then be defined as below some minimum cut-off. For example, if a battery of 1,000 such questions like the one above, asking about many different tenets of Christianity, were given to a person, he/she would then get an overall average "faith score". Let's say it were 8. This person would then have a "stronger faith" than someone with a score of 2. Perhaps the "weak in faith" cutoff would be 7, and so the first person would not be weak and the other person would be.

What's interesting to consider is whether the cut-off can be set at some number other than 10. That is, if one has any doubts about the 1,000 questions, is that person therefore weak in faith? I don't think this is addressed within the Bible itself, but likely is within the history of theology.

I will try to answer your two questions.

1. To your first inquiry of how do we measure faith: we do not, only GOD can do that. It is the Lord's job to judge, we as humans have an insufficient perspective of all things, including ourselves. Therefore, to take it upon ourselves to measure faith as if it could be placed on a scale might be to take the place of God, and thus potentially break the first commandment.

2. To your second of how do we know if someone is weaker in faith we could still fall back on what I already said, but for Paul in Romans 14 (which is listed in another answer) someone is weaker in faith if their conscience is bound and they therefore do not partake in an activity even though it would not be sinful for them. In Romans 14 this is in reference to eating certain foods, and I am sure there are some cultural equivalents today (such as wine, cigars, tattoos, piercings, the cutting of ones hair, not kissing before a wedding night).

Apparently for Paul, the Christian life is intended to be a life of freedom, a life free to live in the love of God and neighbor and less about living under the strict confines of an uber pious morality. Perhaps for Paul, the person who is more free to enjoy every bit of creation is the one who has a "stronger faith."

I hope this helps!

Romans 14 addresses this question in a practical way. It largely assumes that the reader already understands what is meant by weak faith, but the chapter still gives insight into what Paul meant by "weak faith".

(All quotes below are NASB.)

1 Now accept the one who is weak in faith, but not for the purpose of passing judgment on his opinions. 2 One person has faith that he may eat all things, but he who is weak eats vegetables only. 3 The one who eats is not to regard with contempt the one who does not eat, and the one who does not eat is not to judge the one who eats, for God has accepted him. 4 Who are you to judge the servant of another? To his own master he stands or falls; and he will stand, for the Lord is able to make him stand.

Paul mentions the specific case of dietary laws, but a generalization of his view presented here is that the person with weaker faith has a bigger tendency to be legalistic and the person with stronger faith is comfortable with a larger variety of activities. The bottom line though is that neither brother should allow difference in faith to affect their view of the other.

Both in verse 3 above and verse 10 below Paul points out that the person with weaker faith tends to judge others (for their "improper" behavior) while in contrast the person with stronger faith tends to view others with contempt (for their lack of understanding of their freedom).

If you have a tendency to judge others or if you have a tendency to view others with contempt, you need to consider who the ultimate judge is.

10 But you, why do you judge your brother? Or you again, why do you regard your brother with contempt? For we will all stand before the judgment seat of God.

11 For it is written,

“AS I LIVE, SAYS THE LORD, EVERY KNEE SHALL BOW TO ME, AND EVERY TONGUE SHALL GIVE PRAISE TO GOD.”

12 So then each one of us will give an account of himself to God. _{(Emphasis added.)}

So what's the practical application/result of this understanding of "weak faith"?

13 Therefore let us not judge one another anymore, but rather determine this—not to put an obstacle or a stumbling block in a brother’s way.

22 The faith which you have, have as your own conviction before God. Happy is he who does not condemn himself in what he approves. 23 But he who doubts is condemned if he eats, because his eating is not from faith; and whatever is not from faith is sin.

1. Live your life according to the convictions God has given you.
2. Do not judge the convictions of a brother in Christ.
3. Always be willing to yield your behavior to the stronger convictions of a brother so that you will not cause him to stumble.