"He passes on now to lay down a precept especially necessary for the instruction of the Church,---that they who have made the most progress in Christian doctrine should accommodate themselves to the more ignorant, and employ their own strength to sustain their weakness; for among the people of God there are some weaker than others, and who, except they are treated with great tenderness and kindness, will be discouraged, and become at length alienated from religion. And it is very probable that this happened especially at that time; for the Churches were formed of both Jews and Gentiles; some of whom, having been long accustomed to the rites of the Mosaic Law, having been brought up in them from childhood, were not easily drawn away from them; and there were others who, having never learnt such things, refused a yoke to which they had not been accustomed.
Now as man's disposition is to slide from a difference of opinion to quarrels and contentions, the Apostle shows how they who thus vary in their opinions may live together without any discord; and he prescribes this as the best mode,---that they who are strong should spend their labor in assisting the weak, and that they who have made the greater advances should bear with the more ignorant. For God, by making us stronger than others, does not bestow strength that we may oppress the weak; nor is it the part of Christian wisdom to be above measure insolent, and to despise others. The import then of what he addresses to the more intelligent and the already confirmed, is this,---that the ampler the grace which they had received from the Lord, the more bound they were to help their neighbors." -- Calvin's Commentary on Romans, Chapter XIV.1
NB: in the second paragraph, Calvin declares that all strength, all progress in Christian doctrine, and all greater advances that distinguish the strong from the weak, are given by God according to the measure of faith, the proportion that He assigns to each believer. God is the one "making us stronger than others," "bestow[ing] strength." This harks back to Romans 12:3, 6; and the whole fourteenth chapter of Romans fleshes out the exhortations of 12:3-21, according to the appeal of 12:1-2.
A living sacrifice lives not for himself but for God and for others. In 12:3-8, Paul says we are all members of one body, having gifts that vary according to the grace God has bestowed, and so we ought not to think of ourselves more highly than soberly considering that all we have comes from the grace of God. So humility is in order, not haughtiness but associating with the lowly, because that is all that we are: recipients of God's grace. The strong are given their strength to aid the weak, not to oppress or despise them. The body is to work together as one, the members to live in harmony with one another (12:16), to function according to the varied gifting of God, and that with zeal for the Lord and genuine esteem and affection for each other (12:6-13).