Gen 12:1 The LORD had said to Abram, "Leave your country, your people and your father's household and go to the land I will show you. 2 "I will make you into a great nation and I will bless you"
verse 7 The LORD appeared to Abram and said, "To your offspring I will give this land."
Gen 13:4 The LORD said to Abram after Lot had parted from him, "Lift up your eyes from where you are and look north and south, east and west. 15 All the land that you see I will give to you and your offspring forever. 16 I will make your offspring like the dust of the earth, so that if anyone could count the dust, then your offspring could be counted.
Gen 15:1 After this, the word of the LORD came to Abram in a vision: "Do not be afraid, Abram. I am your shield, your very great reward." 2 But Abram said, "O Sovereign LORD, what can you give me since I remain childless and the one who will inherit my estate is Eliezer of Damascus?" 3 And Abram said, "You have given me no children; so a servant in my household will be my heir." 4 Then the word of the LORD came to him: "This man will not be your heir, but a son coming from your own body will be your heir." 5 He took him outside and said, "Look up at the heavens and count the stars--if indeed you can count them." Then he said to him, "So shall your offspring be." 6 Abram believed the LORD, and he credited it to him as righteousness.
Notice that over and over God promises Abram that the number his offspring, from his own body, will be count as the stars of the sky. But so far Abram is childless.
Abram was 75 when God made him this promise. See Genesis 12:4
Now, in Genesis chapter 16, Abram is 86 years old (see Gen 16:16) and God still hasn't given Abram a child. So Abram has waited 11 years and still the promised child hasn't arrived.
Abram probably thought, "God, it took you 6 days to create the earth, how long does it take you to keep your promise?"
Well, after 11 years of no child, Sarah comes up with an idea. Her idea was let's help God keep His promise. Does that sound like faith or does it sound like works? Yes, the latter. Did Abram agree? Yes, let's read this:
Gen 16:1 Now Sarai, Abram's wife, had borne him no children. But she had an Egyptian maidservant named Hagar; 2 so she said to Abram, "The LORD has kept me from having children. (Notice she blames God) Go, sleep with my maidservant; perhaps I can build a family through her." Abram agreed to what Sarai said. 3 So after Abram had been living in Canaan ten years, Sarai his wife took her Egyptian maidservant Hagar and gave her to her husband to be his wife. 4 He slept with Hagar, and she conceived.
Now, was this sexual act, an act of faith or were they trying to help God keep His promise?
No, it wasn't an act of faith, but Abram was still righteous by faith, even though his faith was wavering.
More latter.