Did Paul say Sabbath keepers are weak in the faith?
Emphatically NO.
Weak in the faith comes from here:
Romans 4:18-22 KJV
Who against hope believed in hope, that he might become the father of many nations; according to that which was spoken, So shall thy seed be. [19] And being not weak in faith, he considered not his own body now dead, when he was about an hundred years old, neither yet the deadness of Sara's womb: [20] He staggered not at the promise of God through unbelief; but was strong in faith, giving glory to God; [21] And being fully persuaded that, what he had promised, he was able also to perform. [22] And therefore it was imputed to him for righteousness.
Abraham was not weak in faith, meaning he fully accepted what God said and acted on it. He obeyed, in fact.
Romans 14:1-2 KJV
Him that is weak in the faith receive ye, but not to doubtful disputations. [2] For one believeth that he may eat all things: another, who is weak, eateth herbs.
The vegetarian is weak in the faith because he is worried about eating something that will defile him. Whether this applies to rabbinical fasting days or to abstaining from animal products produced by gentiles (rabbinically prohibited, by the way, not prohibited by God) or meats offered in sacrifice to idols (or a surefire way to avoid doing such a thing by just going vegetarian), the end result is the same. The weak brother believes if he does these things it will be displeasing to God and the community of faith.
Is it weak in the faith for a Christian to abstain FROM SIN? Hardly.
Romans 14:5 KJV
One man esteemeth one day above another: another esteemeth every day alike . Let every man be fully persuaded in his own mind.
Notice, there is no mention of weakness in this passage. If one were to say well it's parallel to the food one so there is still weakness being discussed? Well, following the parallel from earlier in the chapter, the SECOND one mentioned is weak. In this verse it would be the guy who esteems every day alike who is weak in the faith!
But nevermind, because this isn't about the Sabbath anyway. It is about "doubtful disputations" and there is no doubt about the sanctification of the seventh day as the Sabbath. It is about personal estimation of the value of certain days, not about God's estimation of the Sabbath. The Sabbath is never even mentioned in the chapter.
Romans 14:21 KJV
It is good neither to eat flesh, nor to drink wine, nor any thing whereby thy brother stumbleth, or is offended, or is made weak.
Weakness is equated with stumbling and offending. Biblically, stumbling and offending have to do with sinning (transgressing God's commandments, including Sabbath breaking). So making your brother weak is not done by encouraging OBEDIENCE TO GOD, but by encouraging DISOBEDIENCE.
1 Corinthians 8:4-7 KJV
As concerning therefore the eating of those things that are offered in sacrifice unto idols, we know that an idol is nothing in the world, and that there is none other God but one. [5] For though there be that are called gods, whether in heaven or in earth, (as there be gods many, and lords many,) [6] But to us there is but one God, the Father, of whom are all things, and we in him; and one Lord Jesus Christ, by whom are all things, and we by him. [7] Howbeit there is not in every man that knowledge: for some with conscience of the idol unto this hour eat it as a thing offered unto an idol; and their conscience being weak is defiled.
Faith is not here said to be weak, but the conscience. Those whose conscience is weak are those who do not know that idols are nothing and powerless. Kind of like today people think a Buddha statue has some occultic power in itself. Truth is it is nothing, nothing but a memorial to foolishness, if anything. So some were refusing to eat gentile food because butchers and hosts were sanctifying the animal from which it came to pagan gods, and these believers wanted nothing at all to do with idolatry.
1 Corinthians 8:8-13 KJV
But meat commendeth us not to God: for neither, if we eat, are we the better; neither, if we eat not, are we the worse. [9] But take heed lest by any means this liberty of yours become a stumblingblock to them that are weak. [10] For if any man see thee which hast knowledge sit at meat in the idol's temple, shall not the conscience of him which is weak be emboldened to eat those things which are offered to idols; [11] And through thy knowledge shall the weak brother perish, for whom Christ died? [12] But when ye sin so against the brethren, and wound their weak conscience, ye sin against Christ. [13] Wherefore, if meat make my brother to offend, I will eat no flesh while the world standeth, lest I make my brother to offend.
Again, we are not to encourage a brother to eat such meat IF that brother believes it is wrong because of the idol connection. But notice:
1 Corinthians 10:18-31 KJV
Behold Israel after the flesh: are not they which eat of the sacrifices partakers of the altar? [19] What say I then? that the idol is any thing, or that which is offered in sacrifice to idols is any thing? [20] But I say , that the things which the Gentiles sacrifice, they sacrifice to devils, and not to God: and I would not that ye should have fellowship with devils. [21] Ye cannot drink the cup of the Lord, and the cup of devils: ye cannot be partakers of the Lord's table, and of the table of devils. [22] Do we provoke the Lord to jealousy? are we stronger than he? [23] All things are lawful for me, but all things are not expedient: all things are lawful for me, but all things edify not. [24] Let no man seek his own, but every man another's wealth . [25] Whatsoever is sold in the shambles, that eat, asking no question for conscience sake: [26] For the earth is the Lord's, and the fulness thereof. [27] If any of them that believe not bid you to a feast , and ye be disposed to go; whatsoever is set before you, eat, asking no question for conscience sake. [28] But if any man say unto you, This is offered in sacrifice unto idols, eat not for his sake that shewed it, and for conscience sake: for the earth is the Lord's, and the fulness thereof: [29] Conscience, I say, not thine own, but of the other: for why is my liberty judged of another man's conscience? [30] For if I by grace be a partaker, why am I evil spoken of for that for which I give thanks? [31] Whether therefore ye eat, or drink, or whatsoever ye do, do all to the glory of God.
If it is made known that food is offered to idols, we are not to partake because of the idolatry connection. Partaking becomes an act of intentional worship in such a case. So we abstain in such situations.
So that's about it in regard to being weak in the faith. Nothing in scripture suggests Sabbath keeping is a weakness of faith.