Genesis 17:1

And when Abram was ninety years old and nine
Which was thirteen years after the birth of Ishmael last mentioned; so many years more it was before be is expressly told he should have a son by Sarai, or had the promise of Isaac, which was for the trial of his faith; and his age is here observed, that the power of God might be more manifest in fulfilling his promise, and giving him a son by Sarai:

the Lord appeared to Abram;
in a visible manner, in an human form very probably, even the Logos, the Word and Son of God: it seems as if the Lord had not appeared to him since the birth of Ishmael, until this time; and if so, it may be thought to be a correction of him for listening to the voice of his wife in marrying Hagar, without asking counsel of God:

and said unto him, I [am] the Almighty God;
as the Word of God is, as appears by his creation of all things, his in sustaining of them, his government of the church, his redemption of it, and preservation of his people safe to glory, see ( Revelation 1:8 ) ; and this epithet is very appropriate here, when the Lord was about to give out a promise of a son to Abram and Sarai, so much stricken in years. Some render it "all sufficient" {c}, as Jehovah is, sufficient in and of himself, and for himself, and stands in no need of any, or of anything from another; and has a sufficiency for others, both in a way of providence and grace:

walk before me:
not as though Abram had not so walked, or had discontinued his walk before God, but that he would go on to walk by faith in a dependence on him for everything he wanted, both with respect to things temporal and spiritual; and to walk in all his commandments and ordinances, that he either had given, or should give him; and all this as in his presence, and under his watchful eye, that sees and observes all things, and before whom all things are naked and open, as all are to the essential Word of God, ( Hebrews 4:12 Hebrews 4:13 ) ;

and be thou perfect:
upright and sincere in acts of faith, and in duties of religion, and go on to perfection; which though a sinless one is not attainable in this life, is desirable, and is to be had in Christ, though not in ourselves: but here it chiefly denotes an holy and unblamable life and conversation, which though not entirely free from sin, yet without any notorious ones, which bring dishonour to God, and disgrace upon a man's character and profession, see ( Genesis 6:9 ) . This respects not perfection in his body or flesh, as the Targum of Jonathan paraphrases it, through circumcision, by which the Jews ^F4 fancy Abram became perfect, but was not till circumcised.