Which he shed, (or, whom he shed.) In the Greek, the relative may apply either to the "washing" or to the "Spirit;" for both of the nouns -- loutron and Pneuma -- are neuter. It makes little difference as to the meaning; but the metaphor will be more elegant, if the relative be applied to loutron the "washing" Nor is it inconsistent with this opinion, that all are baptized without any distinction; for, while he shews that the "washing" is "shed," he speaks not of the sign, but rather of the thing signified, in which the truth of the sign exists. When he, says, abundantly, he means that, the more any of us excels in the abundance of the gifts which he has received, so much the more is he under obligations to the mercy of God, which alone enriches us; for in ourselves we are altogether poor, and destitute of everything good. If it be objected that not all the children of God enjoy so great abundance, but, on the contrary, the grace of God drops sparingly on many; the answer is, that no one has received so small a measure that he may not be justly accounted rich; for the smallest drop of the Spirit (so to speak) resembles an ever-flowing fountain, which never dries up. It is therefore a sufficient reason for calling it "abundance," that, how small soever the portion that has been given to us it is never exhausted. Through Jesus Christ [1] It is he alone in whom we are adopted; and therefore, it is he alone, through whom we are made partakers of the Spirit, who is the earnest and witness of our adoption. Paul therefore teaches us by this word, that the Spirit of regeneration is bestowed on none but those who are the members of Christ.

Footnotes

1 - "When we wish to ascertain the method of our salvation, we must begin with the Son of God. For it is he who hath washed us by his blood -- it is he who hath obtained righteousness for us by his obedience -- it is he who is our Advocate, and through whom we now find grace -- it is he who procured for us the adoption by which we are made children and heirs of God. Let us carefully observe that we must seek all the parts of our salvation in Jesus Christ; for we shall not find a single drop of it anywhere else." -- Fr. Ser.

Which he shed on us - Greek, "Which he poured out on us" - ἐξέχην exechēn; see the notes at Acts 2:17. The same Greek word is used there as here. It occurs also in the same sense in Acts 2:18, Acts 2:33.
Abundantly - Margin, as in Greek, "richly." The meaning is, that the Holy Spirit had been imparted in copious measure in order to convert them from their former wickedness. There is no particular allusion here to the day of Pentecost, but the sense is, that the Holy Spirit had been imparted richly to all who were converted, at any time or place, from the error of their ways. What the apostle says here is true of all who become Christians, and can be applied to all who become believers in any age or land,
Through Jesus Christ our Saviour - See Notes, Acts 2:33.

Which he shed on us abundantly - Οὑ εξεχεεν· Which he poured out on us, as the water was poured out on them in baptism, to which there is here a manifest allusion; but as this was sometimes only sprinkled on the person, the heavenly gift was poured out, not in drops, but πλουσιως, richly, in great abundance.
Through Jesus Christ - Baptism is nothing in itself; and there had been no outpouring of the Holy Spirit, had there been no saving and atoning Christ. Through him alone all good comes to the souls of men.

Which he shed on us abundantly,.... "Or richly"; either which love he shed abroad in the hearts of those whom he regenerated and renewed by his Spirit; or which water of regeneration, that is, grace, comparable to water, he plentifully shed, and caused to abound where sin had done; or rather whom, or which Holy Spirit, with his gifts and graces, such as faith, hope, and love, and every other, he poured forth in great abundance on them; see Isaiah 44:3
through Jesus Christ our Saviour; the love and kindness of God the Father our Saviour, comes through him; the mercy of God streams through him; the salvation itself is by, and through him; the grace communicated in regeneration and renovation is out of his fulness; the Spirit himself is given forth from him; and every supply of grace, by which the work is carried on, comes out of his hands; and everything wrought in us, that is well pleasing in the sight of God, is through him; and even the gift of God, eternal life itself.

Which--the Holy Ghost.
he shed--Greek, "poured out"; not only on the Church in general at Pentecost, but also "on us" individually. This pouring out of the Spirit comprehends the grace received before, in, and subsequently to, baptism.
abundantly--Greek, "richly" (Colossians 3:16).
through Jesus Christ--the channel and Mediator of the gift of the Holy Ghost.
our Saviour--immediately; as the Father is mediately "our Saviour." The Father is the author of our salvation and saves us by Jesus Christ.